Quote:
Talking with her about a threesome is stupid. Just lead it there and she likely will be down.
This. ^^^
If you were in a committed relationship and you wanted to experiment with swinging and threesomes... definitely talk about it. Communication is key.
But for a girl that you're just fooling around with... You've just gotta set it up so it works out.
Getting both of the girls you are trying to have sex with into the same room alone is a good start. Do whatever you've gotta do to make that happen. Then, just set the sexual tone and let their pussies start dripping over the idea of sharing your cock.
Also, be open to the idea you might have to have a threesome with her and another guy too if she's into that. Keep things fair.