Why is it said that Lord Vishnu has given Lord Shiva his trident when there seem to be only 2 tales in this regard?:
As per Abridged version of Vamana Purana by International Gita Society (IGS), Lord Shiva and Lord Vishnu had to exchange their weapons to kill a demon named Jalodbhav. Here is the complete story (the text in  added by me to clarify):
Narada was surprised and asked Pulastya why Lord Shiva and Lord Vishnu had held Sudarshan chakra and trident respectively. Pulastya explained- "During ancient times, there was a mighty demon king named Jalodbhav. He had pleased Lord Brahma by his severe penance and was blessed by him to become immortal [can only be killed in very rare scenario when Shiva attacks with Chakra and Vishnu attacks with Trishula]. After receiving the boon, Jalodbhav became arrogant and started tormenting the deities and the sages. The deities sought the refuge of Lord Vishnu. They accompanied Lord Vishnu to Himalaya Mountain to meet Lord Shiva. Lord Vishnu and Lord Shiva exchanged their weapons. Both of them went to kill the demon who on seeing them got scared and hid in a nearby river. Lord Vishnu and Lord Shiva waited eagerly for him on the opposite banks of the river."
"After sometime, Jalodbhav came out of the river thinking that both of them must have departed by now. He climbed up the Himalaya Mountain and both the deities chased him. Lord Shiva attacked him with his chakra while Lord Vishnu pierced his trident in his chest. Jalodbhav died and fell down from the Himalaya Mountain." After finishing his story, Pulastya says- "This was the reason why both Lord Vishnu and Lord Shankar had exchanged their weapons.
So after this incident, Vishnu would have given (returned) trishula/trident to Shiva.
There two Puranas mention that Lord Vishnu gave the trishul to Lord Shiva.
In Brahmavaivartana Purana,Prakriti Kanda,Chapter 16
Therefore, lord Siva should go to earth with my trident and should kill him with it.My kavaca is held by that demon in his neck which has made him always victorious on earth.О Brahmana, till such time the kavaca is worn by him, no one on earth can kill him. Therefore I shall beg it from him in the form of an ascetic. The moment the chastity of his wife is broken he will be killed. You have also given him a boon accordingly.For this purpose I shall implant my semen in the womb of his wife and he will meet with his end at the same moment. There is no doubt about it.Thereafter shedding the form of a woman she would become a beloved of mine. Thus speaking lord Visnu handed over the trident delightfully to lord Siva and returned to his abode. Thereafter making Siva to lead the gods, he left the place. All the gods also reached the land of Bharatavarsa.
And in Devi Bhagavatam
Take this My weapon Śūla and go early to Bhārata. Śiva will slay the Dānava by this Śūlāstra. The Dānava holds always on his neck My auspicious Kavaca and will therefore become the conqueror of the universe. No one will he able to kill him as long as be holds the above Kavaca. So, first of all, I will go to him in the form of a Brāhmaṇa and ask from him the Kavaca. O Creator! Thou also didst give him the boon that his death would occur when the chastity of of his wife would be destroyed. I will go and hold intercourse with his wife. Then his death will occur without fail. His wife after her death will come and become My dearest wife. Thus saying, Nārāyaṇa gave over to Mahādeva the Śūlāstra.