1 - Groot: Groot is still alive and still is a Guardian, so he will clearly appear in Guardians 3, it couldn't more obvious.
2 - Thor: James Gunn confirmed the movie takes event after Endgame, so Thor will obviously appear in Guardians 3, he is part of the team now.
3 - Gamora: the movie will clearly show the Guardians searching for her past self.
That would be considered speculation (even though it's pretty much a given it's gonna happen), but this wikia, speculation is frowned upon, and is a no-no. You'd need an official source confirming it, and because that Disney/Marvel/Feige hasn't given out any official statements about Groot, Thor or Gamora yet, we cannot add it to the wikia.
Personally, if it was up to me, I would, but that kind of leniency isn't allowed here.