Guardians of the Galaxy 3 Takes Place After Avengers: Infinity War
"Much of what’s happened in the MCU for the past ten or so years has been leading, in a big way, to the Avengers’ Infinity War," Gunn posted on Facebook. "Guardians of the Galaxy Vol. 3 will happen after all that. It will conclude the story of this iteration of the Guardians of the Galaxy, and help catapult both old and new Marvel characters into the next ten years and beyond. I will be working side-by-side with Kevin Feige and the gang to help design where these stories go, and make sure the future of the Marvel Cosmic Universe is as special and authentic and magical as what we have created so far."
The Avengers: Infinity War is set for a May 4, 2018 release, and the untitled Avengers 4 comes out a year later on May 3, 2019, so more than likely Guardians of the Galaxy 3 will be released some time in 2020.
Marvel Studios does have an untitled movie set for a May 1, 2020 release date, so more than likely that's for Guardians of the Galaxy 3.
And a bonus that Guardians of the Galaxy 3 comes out after The Avengers: Infinity War movies is that similar to the first two Guardians of the Galaxy movies, more than likely the third won't be tied to anything else in the MCU--meaning Guardians of the Galaxy 3 will be independent and standalone.
Also from what James Gunn says above, it sounds likely that Guardians of the Galaxy 3 could lead to further Marvel Cosmic spinoff movies.