With Marvel confirming at San Diego Comic-Con 2019 that Thor: Love and Thunder will arrive before Guardians of the Galaxy 3, the biggest question is why Thor has left the Benatar. Thor 4 will be included in the MCU's Phase 4 while Guardians of the Galaxy 3 will likely be part of Phase 5. Keeping up continuity, especially when it comes to Thor's side of the story, will be essential.
Thor's story is in an interesting place as the MCU heads into Phase 4. At the end of Avengers: Endgame, Thor makes some big changes to his life. He steps down as the king of New Asgard and hands that privilege over to Valkyrie. After doing this, he hops into the Benatar with the rest of the Guardians (which he dubs the Asgardians of the Galaxy) and jets off into space. It's a new beginning for the God of Thunder. Except with his next adventure being a solo Thor 4, there's the question of how to transition him back out of the team and ensure Endgame's future tease doesn't fall flat.
Having the Guardians appear for a brief, pre-title sequence in Thor: Love and Thunder to help get Thor back in the story is the perfect way to make this happen: the Guardians can be kicking the God off the Benatar and sending him back to Earth after getting frustrated with his bravado. That's the exact sort of opening, perhaps even before the Marvel Studios logo, that audiences have come to expect. In fact, it would serve a similar purpose to Thor: Ragnarok's opening ditching his Infinity Stone quest from Avengers: Age of Ultron.
Of course, this doesn't mean that Thor is necessarily absent from Guardians of the Galaxy 3. All that's been confirmed by writer-director James Gunn is that his movie takes place after Thor: Love and Thunder, with no hint of Chris Hemsworth's presence. Considering that Guardians Vol. 3 was originally planned to release in 2020 following Endgame directly, it's possible Thor has a small role to play. Certainly, with the emergence of Jane Foster as the Mighty Thor, Odinson may want to reconnect with his former pals from Avengers: Infinity War. This could be shown in a post-credits scene or a quick scene towards the end showing the team coming to get him back since they'll no doubt need him.
Using the Guardians in this way would help lay some groundwork for Guardians of the Galaxy 3 while also serving the Thor 4 plot. Plus, every MCU movie is made better by at least 20% when the Guardians show up. Whether or not things revert back to the post-Endgame status-quo will have to wait until Thor: Love and Thunder releases in November 2021.
