Ha! Figured out how to get the threesome with Diane and Debby.
Say good night to Diane, then go to Debby's room. Diane will intercept you and ask for some attention instead. Debby catches you in the act, all hell breaks loose, you get sent to bed, Debby has a sit-down chit-chat with Diane, all is forgiven, Debby comes and gets you and brings you back to her bedroom.
If you cum in Debby nothing happens, but if you cum in Diane you'll still get the Wheel of Misfortune. Keep your finger on the Ctrl key when spinning the wheel. If it comes up preggers, hit that Ctrl key immediately and go for another spin. Keep doing it until you're guaranteed she won't get knocked up.