I'll try to keep my excitement to a minimum, because after all, registering a domain mean absolutely squat, especially since there was no official announcement. But let's look at it glass half-full. Avalanche and Square Enix knows fans loved Just Cause 2 and its crazy antics, so naturally I'd like to think a sequel was planned at some point. It's also just under two months to E3, and a Just Cause 3 announcement wouldn't be that out of the question.
If you want, you can check out the WHOIS for justcause3game.com here.
But seriously though, MAKE THIS HAPPEN!