Borderlands 3 looks set to release on September 13, and the PC version is believed to be available only through the Epic Games Store. This is evident from tweets from the Borderlands account, which were online and have been removed.
Twitter user Wario64 posted a video which features Borderlands 3 with an Epic Games Store logo. A Steam logo is missing. Wario64 says the images are from a video ad posted on Twitter. The Borderlands 3 account posted a tweet on Monday stating that the game will be released on September 13. That message has been removed, but can still be seen via the Internet Archive.
It looks like the PC version of the game will be released exclusively on the Epic Games Store. This is probably a temporary deal; for example, the game may still appear on Steam a year later. Epic has recently been making deals with publishers and developers to get games exclusively on its platform in hopes of attracting more users.
Gearbox itself will announce more about Borderlands 3 on Wednesday. Presumably, it will then be officially announced where and when the game will appear. Borderlands: Game of the Year Edition will also be available from Wednesday, an improved version of the first part. It is the first time that the game will be available on the PlayStation 4 and Xbox One. The PC version is a free update for players who already have the existing GotY version.