I had a situation a few years ago where i had 2 girls in my bed and thought it was definately on! I ended up paying too much attention to one of them, the other got pissed off and left, so the one i was trying with got worried about her friend and fucked off too! Ever since i learned about the dual massage i wish i'd tried that to lower everyones inhibitions. To be fair it sounds like u were at a bit of a disadvantage in that they had already talked openly about it, i think its better when you just go with the moment and let it flow. I think it was mystery or someone who said, make it their fantasy not yours, which, given the situation, i guess you just have to get them over the LMR and do a bit of push-pull.
Saying that i'm still awaiting the elusive 3some!