Re: Tyrus Thomas
First of all there is the possibility that Biyombo might not play more than 15mpg in his rookie season anyway. Remember that he is very, very raw. He'll still have to learn basics, and he'll probably have a rough time avoiding fouls in his first season. That should leave enough minutes for all three players. Especially since Biyombo can play a bit of C and Diaw a bit of SF.
But if Biyombo is good enough to play extended minutes, I see him splitting his time between the PF and the C spot. Hey may be a bit short for the C spot, but he is heavy and strong, especially for his age. And we don't have any centers on the team right now, so even if we get one in free agency there is a lot of time to be filled at that position. That should leave plenty of minutes for Tyrus to play minutes at the PF slot.
Boris Diaw's minutes should simply not be a priority at this point. It's a bit of a d1ck move to diminish his role like this in his contract year, but business is business. It's his contract year, he's obviously not coming back after this season and the Bobcats won't win many games with or without him right now. So there is no reason to play him instead of Tyrus or Biyombo.