If she is bi, then you have hit a jackpot havn't you? They are usually open minded and chances of threesomes are highly likely not just possible. All you need to do now is to get her to bite.
STR8 up is not necesssarily the best way to go. Most women if not totally bi, have at least considered or fantasized about other women and are somewhat reluctant to admit it. The trick is implant the seed and wait for a response. Get into a conversation about some other good looking girl and see if she chimes in. Tell her how fucking hot the thought of 2 women together gets you. Sometimes fishing's the best way to go about things...