Seems like Soul Calibur 3 will be a PS2 exclusive, unlike SC2.

Featuring 3 new characters (which will up the total playable characters to 25), a character creation mode where you create your own character (customizing the gender, profession?, weapons), and can participate in World Competition mode (whatever that is?).

Other modes include Soul Arena, and Museum. The Museum mode will include history from its prequels...

The information mostly came from Gamepro magazine, and my sources are listed below.

Links: (In various languages!)
Chinese article
Mag scans
French, but you can babelfish