The Guardians of the Galaxy movies all have complicated and time-consuming sets to build in addition to the pre-visualization work that has to go down before production can even start. A May 2020 release date for Guardians of the Galaxy 3 has already been speculated about, since the previous movie came out in May. May 1st seems like the most logical of the dates, but Marvel also has August 7th and November 6th open for untitled projects. So, Marvel could be going back for an August premiere, like the first Guardians of the Galaxy movie.
As far as what Guardians of the Galaxy Vol. 3 might be about, that's anyone's guess at this point in time. The future of the team will be decided after the conclusion of Avengers 4, which is still over a year away. We'll probably get some hints to who makes it out of the epic battle with Thanos alive from set pictures, but since Guardians movies are usually shot in studios, it may be a little tougher to gauge. Adam Warlock was teased at the end of Guardians of the Galaxy Vol. 2, but James Gunn has not confirmed that the character will show up in Vol.3.
An early 2019 production start for Guardians of the Galaxy Vol. 3 makes a lot of sense, since there's a lot of heavy lifting to be done in the pre and post-production process. James Gunn and crew will need plenty of time to flesh out the digital effects after filming wraps and an early 2019 production start guarantees plenty of time for a 2020 release date. Again, this has yet to be confirmed by Marvel or James Gunn and is an unconfirmed report at this time. You can read the original report via That Hashtag Show.