Sephiroth does not have three forms. He has one, and that is himself. When he was taken over by Jenova, he was given the ability to turn into those three forms, but not normally.

This was all very meticulously covered in an old Sephiroth battle during the second tournament. I think it is like 4 months back in this Forum if you want to reference it. Actually, I'll find it and edit this post for reference now:

http://www.zelaron.com/forum/showthread.php?t=30647