The third game in the Batman: Arkham franchise is official via the cover reveal of the next Game Informer issue. The title is Batman: Arkham origins and as the name implies it's a prequel to the two Rocksteady developed games.
This time around Warner Montreal (responsible for the Wii U port of Batman: Arkham City) is developing, and the release is planned for PC, PS3, Wii U, and Xbox 360 on October 25. A spin-off called Batman: Arkham Origins Blackgate is in the works for Nintendo 3DS and PS Vita.