Adam Warlock is likely to appear in GotG 3, in fact Kevin Feige seems to confirm he will be in Vol 3, but will not be appearing in Infinity War:
http://io9.gizmodo.com/marvels-cosmic-hero-adam-warlock-will-appear-in-guardia-1794510792
Gunn doesn’t explicitly mention the third Guardians film to /Film, but given that he himself is developing Warlock’s cinematic debut and announced earlier this week he would return to write and direct Guardians 3, the logical conclusion is that Adam is being saved for Guardians 3. This was indeed confirmed by Kevin Feige, speaking to our very own Germain Lussier at the same junket today:
[Adam Warlock’s arrival] will not be until Guardians 3. Adam is not a part of Infinity War, which of course he is in the comics, but James [Gunn] did always really like Adam Warlock, and our version of Infinity War is the culmination of all the stories we told up to this point.
James has had ideas to introduce Adam—certainly there were ideas in Vol. 2, maybe even in part one, but part two for sure—which were great, but the way the story was coming together, there just wasn’t time for that.
http://io9.gizmodo.com/marvels-cosmic-hero-adam-warlock-will-appear-in-guardia-1794510792