Square Enix has registered the Just Cause 3 domain which is likely an indication that development could be happening on this title.
The thing is that Avalanche Studios who has developed Just Cause in the past is already working on Mad Max. Given that bit of info it could be that a second team at Avalanche is starting work on Just Cause 3 or maybe another studio or team will be handling the development of Just Cause 3.
Hard to say with more questions than answers, but it would seem that Square Enix could be getting their ducks in a row so to speak for an E3 announcement. We will have to just wait and see as we don’t expect any comment from Square Enix till at least E3.