How shiva became Tripurari ? I have heard that Brahma became charioteer and other deities became chariot? And still they say shiva does not need anything but was playing with the stuff of devas?
Lord Shiva became Tripurai by destroying Three (Tri) cities (Pura) made up of iron gold and silver.
YajurVeda Taittariya Samhita 6.2.3 mentions this story as:
The Asuras had three citadels; the lowest was of iron, then there was one of silver, then one of gold. The gods could not conquer them; they sought to conquer them by siege; therefore they say--both those who know thus and those who do not--'By siege they conquer great citadels.' They made ready an arrow, Agni as the point, Soma as the socket, Visnu as the shaft. They said, 'Who shall shoot it?'  'Rudra', they said, 'Rudra is cruel, let him shoot it.' He said, 'Let me choose a boon; let me be overlord of creatures.' Therefore is Rudra overlord of creatures. Rudra let it go; it cleft the three citadels and drove the Asuras away from these worlds. The observance of the Upasads is for the driving away of foes.
In the eosteric sense Tripura destruction means destruction of three (Tri) places (puras) created by Illusion (Mayasura). It may be Satwa, Rajas, Tamas or Sthula, Sukshma, Karana Sariras or whatever else formed by Maya.
In any case the above story of Veda Samhita is explained in Mahabharat Drona Parva section 102 as:
Formerly, the valiant Asuras had, in heaven, three cities. Each of those cities was excellent and large. One was made of iron, another of silver, and the third of gold. The golden city belonged to Kamalaksha, the silver city to Tarakaksha, and the third, made of iron, had Vidyunmalin for its lord. With all his weapons, Maghavat (Indra) was unable to make any impression on those cities. Afflicted (by the Asuras), all the gods sought the protection of Rudra. Approaching him, all the gods with Vasava at their head, said, 'These terrible dwellers of the triple city have received boons from Brahma. Filled with pride in consequence of those boons, they are greatly afflicting the universe, O Lord of the gods, none, save thee, is competent to slay them. Therefore, O Mahadeva, slay these enemies of the gods: O Rudra, creatures slain in every sacrifice shall then be thine. Thus addressed by the gods, Mahadeva thus accepted their request, moved by the desire of benefiting them, and said, 'I will overthrow these Asuras. And Hara made the two mountains, viz., Gandhamadana and Vindhya, the two poles of his car. And Sankara made the earth with her oceans and forests his battle car. And the three-eyed deity made that prince of snakes, viz., Sesha, the Aksha, of that car. And that God of gods, the wielder of Pinaka, made the moon and the sun the two wheels of that vehicle. And the triple-eyed Lord made Elapatra and Pushpadanta, the two pins of the yoke. And the valiant Mahadeva made the Malaya mountains the yoke, and the great Takshaka the string for tying the yoke to the poles, and the creatures about him the traces of the steed. And Maheswara made the four Vedas his four steeds. And that lord of the three worlds made the supplementary Vedas the bridle-bits. And Mahadeva made Gayatri and Savitri the reins, the syllable Om the whip, and Brahma the driver. And making the Mandara mountains the bow, Vasuki the bowstring, Vishnu his excellent shaft, Agni the arrow-head, and Vayu the two wings of that shafts, Yama the feathers in its tail, lightning the whetting stone, and Meru the standard, Siva, riding on that excellent car which was composed of all the celestial forces, proceeded for the destruction of the triple city. Indeed, Sthanu, that foremost of smiter, that Destroyer of Asuras, that handsome warrior of immeasurable prowess, adored by the celestials, O Partha, and by Rishis possessing wealth of asceticism, caused an excellent and unrivalled array called after his own name, and stood immovable for a thousand years. When, however, the three cities came together in the firmament, the lord Mahadeva. pierced them with that terrible shaft of his, consisting of three knots. The Danavas were unable to gaze at that shafts inspired with Yuga-fire and composed of Vishnu and Soma. While the triple city commenced to burn, the goddess Parvati repaired thither to behold the sight. She had then on her lap, a child having a bald head with five clumps of hair on it. The goddess asked the deities as to who that child was. Sakra, through ill-feeling endeavoured to strike that child with his thunderbolt. The divine lord Mahadeva (for the child was none other), smiling, quickly paralysed the arm of the enraged Sakra. Then god Sakra, with his arm paralysed accompanied by all the celestials, speedily repaired to the lord Brahma of unfading glory. Bowing unto him with their heads, they addressed Brahma with joined hands and said, 'Some wonderful creature, O Brahma, lying on the lap of Parvati, in the form of a child, was behold by us but not saluted. We have all been vanquished by him. We, therefore, desire to ask thee as to who he may be. Indeed, that boy, without fighting, hath with the greatest ease vanquished us all with Purandara at our head.' Hearing these words of theirs, Brahma. that foremost of all persons, acquainted with Brahma, reflected for a moment and understood that boy of immeasurable energy to be none else than the divine Sambhu, Addressing then, those foremost of celestials with Sakra at their head, Brahma said, 'That child is the divine Hara the Lord of the entire mobile and immobile universe. There is nothing superior to Maheswara. That Being of immeasurable splendour who was beheld by you all with Uma, that divine lord, had assumed the form of a child for Uma's sake. Let us all go unto him. That divine and illustrious one is the Supreme Lord of the world. Ye gods, ye could not recognise that master of the universe.' Then all the gods with the Grandsire repaired to that child, endued with the effulgence of the morning sun. Beholding Maheswara, and knowing that he was the Supreme Being, the Grandsire Brahma thus adored him: Thou art Sacrifice, O lord, thou art the stay and refuge of the universe. Thou art Bhava, thou art Mahadeva, thou art the abode (of all things),and thou art the highest refuge. This whole universe with its mobile and immobile creatures, is pervaded by thee. O holy one, O lord of the past and the future, O lord of the world, O protector of the universe, let Sakra, afflicted with thy wrath, have thy grace.'
This story is also described in Karna Parva chapter 34 with details about construction of chariot:
"‘"The gods said, ‘Gathering all forms that may be found in the three worlds and taking portions of each, we will each, O Lord of the gods, construct a car of great energy for thee. It will be a large car, the handiwork of Viswakarman, designed with intelligence.’ Saying this, those tigers among the gods began the construction of that car. And they made Vishnu and Soma and Hutasana the arrow for Sankara’s use. Agni became the staff, and Soma became the head, and Vishnu the point, O king, of that foremost of arrows. The goddess Earth, with her large cities and towns, her mountains and forests and islands, that home of diverse creatures, was made the car. The Mandara mountain was made its axle; and the great river Ganga was made its Jangha; and the points of the compass, cardinal and subsidiary became the ornaments of the car. The constellations became its shaft; the Krita age became its yoke; and that best of Snakes, viz., Vasuki, became the Kuvara of that car. Himavat and Vindhya mountains became its Apaskara and Adhishthana; and the Udaya and the Asta mountains were made the wheels of that car by those foremost ones among the gods. They made the excellent Ocean, that abode of the Danavas its other axle. The seven Rishis became the protectors of the wheels of that car. Ganga and Sarasvati and Sindhu and the Sky became its Dhura; all the other rivers and all the waters became the chords for binding the several limbs of that car. Day and Night and the other divisions of time such as Kalas and Kasthas, and the Seasons became its Amukarsha. The blazing planets and the stars became its wooden fence; Religion, Profit, and Pleasure, united together, became its Trivenu. The herbs and the creepers, decked with flowers and fruits, became its bells. Making the Sun and the Moon equal, these were made the (other two) wheels of that foremost of cars. Day and Night were made its auspicious wings on the right and left. The ten foremost of snakes having Dhritarashtra for their first, all exceedingly strong, formed the (other) shaft of that car. The Sky was made its (other) yoke, and the clouds called Samvartaka and Valahaka were the leathern strings of the yoke. The two Twilights and Dhritri and Medha and Sthiti and Sannati, and the firmament bespangled with planets and stars, were made the skins for covering that car. Those Regents of the world, viz., the Lords of the gods, of the waters, of the dead, and of treasures, were made the steeds of that car. Kalaprishtha, and Nahusha, and Karkotaka, and Dhananjaya and the other snakes became the chords for binding the manes of the steeds. The cardinal and the subsidiary directions became the reins of the steeds of that car. The Vedic sound Vashat became the goad, and Gayatri became the string attached to that goad. The four auspicious days were made the traces of the steeds, and the pitris presiding over them were made the hooks and pins. Action and truth and ascetic penances and profit were made the chords of that car. The Mind became the ground upon which that car stood, and Speech the tracks upon which it was to proceed. Beautiful banners of various hues waved in the air. With lightning and Indra’s bow attached to it, that blazing car gave fierce light. That space of time which, on a former occasion, had, in the Sacrifice of the high-souled Ishana, been fixed as a Year, became the bow, and the goddess Savitri became the loud-sounding bow-string. A celestial coat of mail was made, decked with costly gems, and impenetrable and effulgent, sprung from the wheel of Time. That golden mountain, viz., the beautiful Meru, became the flagstaff, and the clouds decked with flashes of lightning became its banners. Thus equipped, that car shone brilliantly like a blazing fire in the midst of the priests officiating at a sacrifice. Beholding that car properly equipped, the gods became filled with wonder. Seeing the energies of the entire universe united together in one place, O sire, the gods wondered, and at last represented unto that illustrious Deity that the car was ready. After, O monarch, that best of cars had thus been constructed by the gods, O tiger among men, for grinding their foes, Sankara placed upon it his own celestial weapons Making the sky its flagstaff, he placed upon it his bovine bull. The Brahmana’s rod, the rod of Death, Rudra’s rod, and Fever became the protectors of the sides of that car and stood with faces turned towards all sides. Atharvan and Angirasa became the protectors of the car-wheels of that illustrious warrior. The Rigveda, the Samaveda, and the Puranas stood in advance of that car. The histories and the Yajurveda became the protectors of the rear. All sacred Speeches and all the Sciences stood around it, and all hymns, O monarch, and the Vedic sound of Vashat also. And the syllable Om, O king, standing in the van of that car, made it exceedingly beautiful. Having made the Year adorned with the six seasons his bow, he made his own shadow the irrefragable string of that bow in that battle. The illustrious Rudra is Death’s self. The Year became his bow; Kala Ratri the Death-night therefore, which is Rudra’s shadow, became the indestructible string of that bow. Vishnu and Agni and Soma became (as already said) the arrow. The universe is said to consist of Agni and Soma. The universe is similarly said to consist of Vishnu. Vishnu is, again, the Soul of the holy Bhava of immeasurable energy. For this the touch of that bow-string became unbearable to the Asuras. And the lord Sankara cast on that arrow his own irresistible and fierce wrath, the unbearable fire of anger, viz., that which was born of wrath of Bhrigu and Angirasa. Then He called Nila Rohita (Blue and Red or smoke)—that terrible deity robed in skins,—looking like 10,000 Suns, and shrouded by the fire of superabundant Energy, blazed up with splendour. That discomfiter of even him that is difficult of being discomfited, that victor, that slayer of all haters of Brahma, called also Hara, that rescuer of the righteous and destroyer of the unrighteous, viz., the illustrious Sthanu, accompanied by many beings of terrible might and terrible forms that were endued with the speed of the mind and capable of agitating and crushing all foes, as if with all the fourteen faculties of the soul awake about him, looked exceedingly resplendent. Having his limbs for their refuge, this entire universe of mobile and immobile creatures that were present there, O king, looked beautiful, presenting a highly wonderful appearance. Beholding that car, duly equipped, he cased himself in mail and armed himself with the bow, and took up that celestial shaft born of Soma and Vishnu and Agni. The gods, O king, then commanded that foremost of celestials, viz., Wind, to breathe after that puissant Deity all the fragrance that he carries. Then Mahadeva, terrifying the very gods, and making the very Earth tremble, ascended that car resolutely. Then the great Rishis, the Gandharvas, those throngs of gods and those diverse tribes of Apsaras began to praise that Lord of the gods while he was about to ascend that car. Adored by the regenerate Rishis, and praised by the eulogists and diverse tribes of dancing Apsaras well- versed in the art of dancing, that boon-giving lord, armed with scimitar and arrow and bow, looked very beautiful. Smiling, he then asked the gods, ‘Who will become my driver?’ The gods answered him, saying, ‘He whom thou wilt appoint, will, O Lord of the gods, without doubt, become thy driver!’ Unto them the god replied, ‘Reflecting yourselves, without delay make him my driver who is superior to me!’ Hearing these words uttered by that high-souled Deity, the gods repaired unto the Grandsire and inclining him to grace, said these words, ‘We have accomplished everything, O holy one, that thou hadst ordered us to do in the matter of afflicting the foes of celestials. The Deity having the bull for his mark has been gratified with us. A car hath been constructed by us, equipped with many wonderful weapons. We do not, however, know who is to become the driver of that foremost of cars. Therefore, let some foremost one among the gods be appointed as the driver. O holy one, it behoveth thee to make true those words that thou, O lord, hadst then said to us. Before this, O god, thou hadst even said to us that thou wouldst do us good. It behoveth thee to accomplish that promise. That irresistible and best of cars, that router of our foes, hath been constructed out of the component parts of the celestials. The Deity armed with Pinaka hath been made the warrior who is to stand on it. Striking the Danavas with fear, he is prepared for battle. The four Vedas have become the four foremost of steeds. With her mountains, the Earth has become the car of that high-souled one. The stars have become the adornments of that vehicle. (As already said) Hara is the warrior. We do not, however, see who is to become the driver. A driver should be sought for that car who is superior to all these. Equal to thee in importance is that car, O god, and Hara is the warrior. Armour, and weapons, and bow, these we have got already, O Grandsire. Except thee, we do not behold any person that can make its driver. Thou art endued with every accomplishment. Thou, O lord, art superior to all the gods. Mounting upon that car with speed, hold the reins of those foremost of steeds, for the victory of the celestials and the destruction of their foes.’ It has been heard by us that bowing with their heads unto the Grandsire that Lord of the three worlds, the gods sought to gratify him for inducing him to accept the drivership.
In this way Lord Shiva became Tripurantaka/Tripurari (destroyer of three cities ). The question of why he was helped by Other Gods is like why Lord Rama was helped by Vanara Sena in Ramayana ? In fact the whole story of chariot construction and position of God's in it has deep Yogic significance. They are just like Path of Kundalini Awakening.