James Gunn said that all the Guardians are going to be in Vol 3. And at least with Spider-Man 2 they've said it takes place almost immediately after the 4th Avengers. I doubt the others take place before it. The writers said that there will be a cost to undoing Infinity War's ending. I think the simplest explanation will be correct, someone like Iron Man or Captain America use the Gauntlet to fix everything but it kills them.