I've had mixed threesomes with varying success. Doesn't seem to be much difference between getting the GG first and letting her pick the LB, or getting the LB first and letting her pick the GG. Either way is a bit of a crap shoot; sometimes you win big, and sometimes you crap out.
One of the best threesomes I ever had happened quite by accident at the Penthouse Hotel. After a few drinks in Obsession, ran out to see what the night had in store for me. An old friend spotted me on the street, and we went for dinner and drinks. We went back to the room where the GG did her best to show how much she missed me, when all of a sudden there was a knock on the door. It was the LB I had over-tipped earlier. Things started slow, but it turned out to be a rather good night. I really should write about it some time as it's been a while since I've done a trip report.
One way I wouldn't recommend is picking up a girl on Beach Road and having her call her friend. A guy I know tried that, and he woke up with the girls gone - as well as his backpack!