Not even announced, can't wait for it though. The last I heard some Sega guy said there was room in the world for a 3rd one but not sure when. Also there is that possibly compilation title scheduled for ps2?
You should give #2 another shot, I'm going through the single player again trying to do the extra and master levels on expert, and I think #1 definately had the better single player game (mainly due to the simple yet difficult of levels and no buying extra lives with play points), but the minigames are pretty good.
My dream for monkey ball 3 is to have a skill mission, similar to monkey ball 1 single player and a puzzle mission, with switches etc and things like sticky ball where the monkey can roll on the underside of platforms etc.