Big rumors on SA3 here, seems Zifei (Sonic HQ Creator) has some info:

The game’s playable characters are Sonic, Shadow, Tails, & Knuckles. All stages will be optimised for Sonic and Shadow. Tails and Knuckes will use these stages in their their respective unique ways.

Next step in camera system evolution or as Naka-san bluntly put it: “The best camera system ever seen in any platformer ever.”

For GameCube and Xbox.

The ‘Stadium will try and get to the bottom of these rumours, to see if they really are what they seem.

Source: Sonic HQ News