There are. Look at the four player rules option, and that will dictate how to also split the Free People into two teams. So, if you'd rather do a three player game with two FP players, you can do so. Easy-peasy.
Page 46, rules for the Three-player game:
"..there is only one Free Peoples Player.."
The rulebook bears no mention of only one Shadow Player. I thought this may have been due to one reason or another, intentionally.