The following article contains spoilers for Avengers Endgame.
Now that James Gunn has been reinstated to his rightful place as director of the third and presumably final Guardians of the Galaxy, we now know when filming is set to start. In an article from The Hollywood Reporter, Marvel is planning to shoot Black Widow, The Eternals, and Shang-Chi this year, and Guardians of Galaxy will start filming sometime next year.
At this stage, it is a bit vague as the when in 2020 Vol.3 will start production, as James Gunn booked another high-profile gig during his hiatus from Marvel. Warner Bros wasted no time acquiring the talents of Gunn and snapped him up to helm their sort of reboot (but kind of not) of Suicide Squad. Production is due to commence this September for an August 2021 release. The original plan would have seen filming on Guardians 3 underway right now, but when it went on indefinite hiatus after Gunn's dismissal, all the cast were released from their commitment to shoot the movie when it was initially scheduled. It is believed that Marvel wants to get the cameras rolling as soon as possible, but mid-2020 seems like a safe bet. Now, we are going to get into Avengers Endgame spoilers here, so this is your last chance look away.
Without being too specific, the Guardians picked up a new member during the close of the movie in the form of Thor. It remains unclear if The God of Thunder was signing up full time, or just grabbing a free ride. Like just about everybody in the audience (and indeed, the world) I am hoping for the former and Thor plays a significant role in Guardians 3. It is worth noting that Chris Hemsworth renewed his contract with Marvel back in 2017 for an undisclosed number of films that would take him well beyond Endgame. There have also been rumors of a fourth Thor movie, but Marvel has been uncharismatically secretive about confirming any details about Phase 4.