To me,
Suzanne sounds more dated than
Susan. Even though
Susan was more popular, I don't know of any. I know of several Suzannes and they're all over the age of 50.
Susan and
Susannah both sounds softer than
Suzanne to me.
I strongly prefer
Susannah to both
Susan and
Suzanne.
Suzanne is my least favorite of the three, probably because of its harsh sound.