Dangan Ronpa - Ultra Despair Girls has been announced for a Western release this Autumn.
This game takes place between Dangan Ronpa 1 and 2, and it is likely that looking at any screenshot will spoil you if you haven't finished Dangan Ronpa 1, since one of the surviving characters from 1 is the secondary main character in this game. As it does take place before Dangan Ronpa 2, you'll probably be okay looking at pre-release material if you haven't finished Dangan Ronpa 2, but I'd personally recommend that you at least still finish the first case.
Unlike Dangan Ronpa 1 and 2, this game is a puzzle-shooter; you have a megaphone that you use to shoot Monokuma's with. The basic plot is that the younger sister of Makoto, Komaru, is forced, by a group of children, known as the Warriors of Hope, to play a game for her survival in a place called Towa City.
Just two things to point out:
Although this game is an interquel, and is a puzzle-shooter, rather than murder-mystery, the story of this game is still very important, and provides some foundation for Dangan Ronpa 3.
This game has considerably more "fan-service" and darker themes than the previous games.