Avengers: Endgame spoilers ahead. Going into Avengers: Endgame, it was unclear who would live and who would die. (That said, it was clear that a lot of the people who had previously died would live. Funny how that works.) Ultimately, two of the original Avengers died; one went back in time and lived an entire life, and three... well, TBD. Thor, for example, left New Asgard in the hands of Valkyrie and took off with the Guardians, who he originally met back in Infinity War. Does that mean that Thor is in Guardians of the Galaxy Vol. 3? It would be another huge twist to his arc, but there are a lot of factors involved.
After the defeat of Thanos, the six original Avengers are left in very different places. Black Widow and Iron Man have died. Captain America is an old man and passes his shield on to Falcon. Hulk and Hawkeye are... sad about their friends dying. Thor tells Valkyrie that she can be the new king.
But what does that leave Thor to do? Apparently, board Star-Lord's ship with the rest of the Guardians. He gets into a little discussion with Quill about who the real leader of the gang is — similar to how they interacted when Thor joined up with the Guardians in Infinity War — and that's the end of Thor's Endgame story.
But, considering he is on the ship and Guardians of the Galaxy Vol. 3 is happening... is Thor just going to stick around? It will probably depend on what Hemsworth wants to do, which could depend on what Thor: Ragnarok director Taika Waititi wants to do, which depends what Marvel wants to do.
Hemsworth has expressed that he liked the direction Waititi took Thor in so much that'd he'd be open to doing more films with him. "We gave cheers to Thor 3," the actor told USA Today of spending New Year's Eve with Waititi in January 2018. "But it also was like, 'What could we do with four? We could do this and this.' We were throwing ideas around. But also, just to work together again. That was one of the most enjoyable creative partnerships I’ve ever had." USA Today reported at the time that Hemsworth's Marvel contract was up after Endgame, which was then referred to simply as Avengers 4.
And while Marvel is keeping everything about future plans for Phase 4 of the MCU under wraps, Tessa Thompson, who played Valkyrie in Ragnarok and in Endgame recently told the Los Angeles Times, "I heard that a pitch has happened for [another Thor film]. I don't know how real that intel is, but I hear that the pitch has happened. I think the idea is Taika [Waititi, who directed Ragnarok] would come back."
So, if Waititi comes back and Hemsworth comes back... well, things could go still either way as far as Thor appearing in the third Guardians of the Galaxy movie. For another Thor movie to happen, Thor will have to have his own stuff going on, which means the Guardians would have to drop him off at some point. So, him appearing in Guardians of the Galaxy Vol. 3 would depend on whether that's shown on screen or not. Also, the order in which these movies would come out matters. If Thor 4 was first, then Thor probably wouldn't be in the next Guardians movie, because he'd no longer be with them. If the Guardians movie is first, maybe Thor makes an appearance at the beginning or maybe it's just explained where he went.
Either way, you probably shouldn't expect Thor to be a main character in the next Guardians movie, because while the hammer-wielding hero and the group of outer space outcasts overlap in Avengers movies, they're really two separate entities. It'd be odd to have Thor take over a Guardians movie, no matter how much he might want to be in charge of Quill's ship.