Williams is not going to be in the rotation. None of the other 4 bigs are likely to clear 30 mpg.
Perk is going to lose 4th quarter mins so he will go back to about 25-28 mpg.
It is difficult to play more than half of the game rotation wise when you come off the bench. It is more than likely that Sheed comes in at about 24-25 mpg.
At that point I think KG will be close to 30 and Baby close to 20.
As far as using 3 bigs together, it will be situational but clearly there will be times where it may be necessary as it may be advantageous. TOR, CHA, CLE as well as other teams out West can put Pierce at significant disadvantage unless he can slide to SG. Just because Baby is on the floor doesn't mean he has to guard the SF it can be Garnett, Wallace, or Perkins who are matching up on the opponents SF.