According to both Kotaku and IGN, Avalanche Studios is currently rumored to be officially revealing Just Cause 3 as early as this year's E3 event with a mid-2015 release date planned.
A recent tweet from Kotaku columnist Superannuation stated Square Enix had purchased the rights for the domain name earlier in the week. He speculated Square Enix might likely be debuting the game during this year's E3 event.
Square Enix's corporate domain registrar registered a Just Cause 3 domain this week. Guess we'll see that at E3. http://t.co/5NSNpTS5nR— superannuation (@supererogatory) April 24, 2014
According to a recent report from IGN, the game is also rumored to be planning for a June 2015 release based on a recent update from Avalanche Studios founder Christofer Sundberg's LinkedIn page. The update teased the studio is currently developing an unannounced sandbox game for a "very well-known game series" for next gen consoles and to be released by mid-2015.