Kingdom Hearts 3 may be launching during the third quarter of 2016.
A Reddit post by user Callum McGaw appears to show the listing of the game's launch window in the latest issue of Play magazine. While the game is coming to both PS4 and Xbox One, the magazine only mentions a PS4 version but that's because it's a PlayStation magazine.
As for the release date, it may be simply be a holding date but it's always possible Play let it slip early, how they would know is anyone's guess. Play does not that "We don't think we'll be seeing the game this year," suggesting it is indeed a placeholder.
The Play article on Kingdom Hearts 3 notes that once Final Fantasy 15 ships we'll likely start to hear a lot more about the game as Square Enix's PR machine moves to focus on the title.
If the magazine is right, expect to Kingdom Hearts 3 some time around September 2016. Tetsuya Nomura confirmed in June that a release for the game has been decided, but Square Enix isn't ready to share it just yet.