I think telling the story of "Knightfall" would really only require two movies. The first movie would deal with introducing Bane and Jean-Paul Valley, Bane breaking all the freaks out of Arkham, Batman busting them, and then Bane breaking Batman's back.
Movie two would deal with Batman passing the mantle of the Bat to Valley, his rise as Batman, him busting Bane, and then going over the edge. Meanwhile, Bruce would take a soul-searching journey around the world, rebuilding himself mentally and physically. When he's ready, he can return to Gotham and have his final showdown with Valley. After beating him, the movie can end with Batman sneaking into Arkham and putting a beating on Bane to regain his spot as king of Gotham.
Both movies would be long, but I think it could be pulled off.