^ +1
My advice would be to have a mental list of positions or scenarios that you want to be sure to try. Don't be shy! As Citizen said don't be afraid to guide the action. Unless you have multiple hours scheduled the time is going to fly so you want to make sure you run through your checklist of threesome highlights
The best doubles sessions for me personally are those in which the girls are really into the girl-on-girl action not just going through the motions. Regardless, if its your first one you'll likely enjoy yourself immensely no matter what happens!