It's coming to PS4 and Xbox One in Japan and Asia on January 25, 2019, followed by North America and Europe on January 29.
No mention has been made of a release of the game on Windows PC, but with the recent porting of Square Enix's Final Fantasy XV, it's not necessarily out of the realm of possibility. The publisher has their own conference at E3 this year, and the release date for the game was expected then. Yes, the company finally announced when we can expect Kingdom Hearts 3 to release, and it'll be here early next year.
The release date puts Kingdom Hearts III nearly 2 years exactly after the previous installment in the series, the mini-episode Kingdom Hearts 0.2 Birth by Sleep -A Fragmentary Passage -.
Microsoft showed off a new world for Kingdom Hearts III during its Electronic Entertainment Expo event today in Los Angeles. At the same time, Square Enix will also unveiled a number of new trailers for the game at E3 2018 later this week.