I don’t like to assume things, but I’m getting the impression that you’re a straight male? I don’t know if that would change my advice at all, but for future reference it would be convenient to have some sort of sense of who I’m talking to, in case it’s relevant.
Anyway, person, look, your fantasies are your fantasies and that’s fine, it’s not my place to judge, but you can not expect me to predict the sexual politics of your bisexual girlfriend. For all I know there is no possible way for you to pull this off because she may not at all be open to that. As far as I know being attracted to both sexes doesn’t automatically predispose you to ménage à trois. Maybe she’s a one person at a time kinda gal? Maybe not. So I can not, and in fact I refuse to, attempt to tell anyone how to get anyone to do anything of a sexual nature. I don’t think I have the authority, nor the right. I’m not Maxim magazine.
In short, don’t ask strangers, ask her. I don’t know what you guy’s relationship is like but perhaps it’s possible for the two of you to have an open discussion about each other’s inclinations and fantasies and what not? Try it, maybe?
p.s. I just woke up, so I’m sorry if this was a boring answer or had dumb typos.