Why are guys so fucking insecure about themselves these days?
Just be a fucking man and be straight up with her. Ask her flat out if she's seeing someone else, tell her you don't date women who are dating multiple guys at the same time and to give you a call when she's done dating the other guy. Don't talk to her, don't message her, don't e-mail her and within 2 weeks she'll be all over your nuts. If not, there's plenty more out there. Have some confidence in yourself about the whole thing and she'll see that, women drool over confident men, but you gotta be confident in yourself. Don't be a dick about it, the more non nonchalant the better. If she still wants to talk or "be friends" while she's dating this other dude, tell her you aren't looking for a friend and whatever you do don't keep in touch with her, let her come to you.
You obviously aren't comfortable with the situation of her dating another guy or you wouldn't have made this thread. Just tell her what's up and let her come to you when she's done with the other guy. You wont' have to try to fuck her because she'll be all over you.