In Guardians of the Galaxy Vol. 2, you see Adam’s cocoon which was created by Ayesha who is a Sovereign species. Ayesha created Adam so he could destroy the Guardians. What’s also important to know is that Guardians of the Galaxy 2 takes place in 2014 and Endgame takes place in 2023. By the time Guardians of the Galaxy Vol. 3 comes out, Adam Warlock would have been born already.
Assuming Adam Warlock appears in Guardians of the Galaxy Vol. 3, I can think of 3 ways they could go with him:
- Adam joins the Guardians in the end of the movie.
- Adam fights the Guardians and dies.
- Adam realizes there’s more to life and tries discover his place in the universe which can lead to more appearances in the MCU like the Fantastic 4, Eternals, and possibly be a major player for Secret Wars.