I suggest you to read more about the concept of Trimurti in Hinduism.
The puranas tried to convey the esoteric truths of the Veda in a popular manner. In the process Puranas elevated some Vedic gods by endowing them with virtues, which they loved to see; while at the same time they relegated some other Vedic gods to secondary status.
For instance, Bŗihaspathi, Brahmaņaspathi and Brahma were the three major gods of Rig-Veda; a large number of riks are in honor of these gods. In the Rig-Veda, Brahmaņaspathi/Bŗihaspathi is god of a very high order. There are over one hundred riks in praise of these two deities, giving a picture of their powers and personalities. However, the statuses of these Vedic Gods underwent a huge change in the Puranas; when new set of gods that emerged by the permutation and combination of their own (Vedic gods) powers replaced them. The new gods took over and the old gods were virtually forgotten.
Brahma: The concept of Brahma as the creator in the purāņa is derived from the Brahmaņaspati/Bŗhaspati of the Rig Veda where they are the creators through the power of the Word. Puranas however denied Brahma proper worship.Between these two stages, Brahma is associated with the power to give a verbal identity to a thought. He is the creator and gives form to the formless.
In a passage of the Rig Veda, Vac is praised as a divine being. Vac is omnipotent, moves amongst divine beings, and carries the great gods, Mitra, Varuna, Indra and Agni, within itself. “All gods live from Vac, also all demigods, animals and people. Vac is the eternal being; it is the first-born of the eternal law, mother of the Vedas and navel of immortality.” The reason, the Vedic rishis paid such glowing tributes to Vac was perhaps because they attached great importance to intelligent communication through speech and to its purity.
In the later parts of the Rig Veda, Brahman (one of the three distinct voices in the Soma sacrifices) is associated with word without which speech is not possible. Brahma (word) and Vac (speech) are partners working towards good communication, spread of knowledge and for the fulfillment of the devotees’ aspirations. If word is flower, speech is the garland. If Vac is the weapons, it is Brahman that sharpens them. In Rig Veda the Vac-Brahman relation is a “growing partnership” (RV 10.120.5, and 9.97.34)
Vishnu: (the pervader) Vishnu initially had a lower position to that of Indra. He is the younger brother of Indra. In the Rig-Veda Vishnu is described as living and wandering on the mountains. He is one of the celestial gods and one of the Adithyas. He resembles Surya and has rays in his appearance.
He later evolved into the most signifificant God and Godhead. The ‘Vishnu Sukta‘ of the Rig Veda (1.154) mentions the famous three strides of Vishnu. It said that the first and second of Vishnu’s strides (those encompassing the earth and air) were visible and the third was in the heights of heaven (sky). The second mantra of the ‘Vishnu Sukta‘ says that within the three vast strides of Vishnu all the various regions of the universe live in peace.
Yaskacharya, in his Nirukta, defines Vishnu as ‘Vishnu vishateh; one who enters everywhere’, and ‘yad vishito bhavati tad vishnurbhavati; that which is free from fetters and bondages is Vishnu.’ Vishnu is also characterized, as ‘veveshti vyapnoti vishvam yah; the one who covers the whole universe, or is omnipresent. In other words, Vishnu became the omnipresent dimension of the supreme Lord.
With the advent of the golden age of the puranas in the Gupta period, the transformation of Vishnu into a supreme Godhead was complete. The virtues and glory of the Vedic Indra and Surya were transferred to puranic Vishnu. At the same time, the Indra was demoted to a demigod, stripped of his power and glory. Indra’s status in puranas is pathetic and he is flawed by envy, greed and other human failings. How sad!
In this process, Vishnu, in place of Indra, became the lord of the universe. The attributes and titles that once applied to Indra were now employed to describe Vishnu. Now, Vishnu (not Indra) is the omniscient and omnipresent Godhead; he is ‘ashrutkarna; whose ears hear all things; and “Svayambhuva” meaning ‘Self-existent’ or ‘Self manifested’
The Bhagavata Purana states that Yajna (Indra) took incarnation as Svayambhuva Manu. That Indra was Vishnu (as Svayambhuva). Vishnu in turn becomes Dhanvantri the divine healer, Prithu the King and the Rishis such as Kapila. His later Avatars are celebrated in various Puranas. On his association with Narayana, he is The Supreme Lord of the universe.
Rudra: In Rig Veda, Rudra is one of the intermediate level gods (antariksha devata) and is celebrated in three or four hymns and described as a fierce, armed with bow and arrows. He is endowed with strong arms, lustrous body and flowing golden hair. He is not purely benefic like other Rig Vedic gods, but he is not malevolent either. He punishes and at the same time rescues his devotees from trouble. He is the Shiva the auspicious one.
In Puranas, he becomes one of the Trinity and is the destroyer. He is the Lord of the universe, the cosmic dancer, the Supreme yogi and master of all yogis.
Vedic Rishi Vamadeva merges into to become one of five faces of Lord Shiva and the aspect of Vama or “preserver” associated with the element of water.
He is at his benevolent best when his consort Uma accompanies him. He is Sowmya (sa uma)
“The Indian mythology was (is) not a static affair, neither was it a luxury. It was linked with the vital spiritual urges and needs of the people, who projected their most haunting dreams, hopes and cravings into their mythys.The changes were not wrought overnight nor was it easily. From the earliest times, the pantheon is the product of a continual clash and friction, not only with gods of other ethnic groups, but among those of various clans and families of the Indian society. Each family seems to have had its preferences for its own set of gods. Those gods who could represent larger segments of life and experiences, who could mobilize greater strength and significance, and later, who could annex other gods by virtue of their greater potentialities grew, while others faded out.
The very fact of the gods changing – growing or diminishing in significance – is a proof of the continual influx of new ideas and a creative conflict with existing ideas.
…..In this period of transition, popular sectarian gods were gradually replacing the older Vedic gods. This new approach to the gods remodeled their characters. The gods which adopted themselves best to the changing needs of times survived. One way they did was by shedding their Vedic characteristics which were rather unsuitable. And, another was by aligning with tutelary gods that were already being worshipped.
Only those gods could adapt themselves who had been ‘minor’ in Rig-Veda, who did not have a detailed profile, i.e. those whose personalities were rather sketchy and suggestive, and could be filled and enriched with suitable traits. Gods like Asvins whose characters , functions and achievements had been too vividly described in Rig Veda to afford introduction of new traits were found unsuitable and quietly dropped by the Purana (epic) literature. On the other hand, gods who were too transparently the personification of natural phenomena could not be transformed into popular powerful gods. Thus, Agni, Vayu, Mitra, Varuna, Parjanya, Surya, Soma, Savitr and Ushas had to give place to the new gods. Similarly, gods whose profile was too dim and had little potential for growth just faded out: E.g. Pushan, Bhaga, Aryaman, Daksha, Amsha, Dayus, and Vivastvat etc.
Only those Vedic gods whose characters were not explicitly known, and who offered significant traits to be developed into rich and complex mythology survived and flourished. For instance; Vishnu and Rudra were minor gods, but their profile indicated traits which could be expanded and enriched veraciously. Let’s take the case of Vishnu; he had the nucleus of ‘tri-pada-vikrama’ the collasal figure of measuring the universe with his three enormous strides; his solar nature; lustrous body; his friendship with Indra; vague references to his unparallel valor;– all these were excellent material for developing him into concrete mythological supreme god…From Indra he imbibed the demon-killing valor; from Surya and Savitr the brilliance and sheen associated with gold; from Mitra the kindly , compassionate and benevolent attitudes towards all existence; and , from Bhaga the fortune bestowing generosity. From solar gods in general he inherited associations with Devayana; and consequently his roles as a savior……The component Vedic gods disappear one after another, after bequeathing their virtues to their successor. They last only so long as their living trait remains relevant to the spiritual needs or material aspirations of the society. “
Excerpts from ‘The Indian Theogony’ by Dr. Sukumari Bahttacharji (Cambridge University Press, 1970)