Wes Craven revealed this on his official site. It appears Kelly Rutherford, from Melrose Place, will appear in Scream 3. Also, this confirms that Liev Schreiber is indeed in Scream 3.
News here: last night ended the 3rd to last day of shooting, ending Liev Schreiber's days on the pic. We've already finished Neve Campbell, David Arquette, and most of the others. Courteney Cox and Parker Posey work Tuesday to end their work, and we have one last day, Wednesday, with Kelly from Melrose Place. Then that's it!
Also, the release date has officially been pushed back to Feb 7, 2000, due to the crowded slate of December - when it seems everybody and his brother is releasing a big film.