Ashley Tisdale and Vanessa Hudgens have both signed on to do High School Musical 3. Corbin Bleu and Monique Coleman are in final negotiations, per their publicists, and Zac Efron is expected to return, though he hasn't made a deal as of now.
All cast members are expected to receive a substantial pay increase from the previous movie. No word on Lucas Grabeel's return. And no word on if we can expect to see Zac in manpris again.