Obviously this whole thing is a sex up for a three way. Lucky you...if you play your cards right.
Anyway, that's what's going on, no matter what anyone says.
After you get them in the car, make sure they're buckled and stuff, just mention casually, "I know we're all a little nervous about this, well I know I am, but let's just relax and have fun. If you both behave, don't worry, I have two condoms with me."