You have a delicate line to walk here. On the one hand, you need to take her at her word about wanting this. She may be young but she's mature enough to make her own decisions and it would be foolish (and disrespectful and hurtful!), to push her away because she's "too good to be true" or because you think she can't possibly want what she says she wants.
On the other hand, it sounds like you do have a lot more experience and perspective than she does, and it's your responsibility to help her make sure she's making informed decisions and thinking everything through. I would talk to her frankly about the potential pitfalls of triads and let her know that she is under no obligation to like you both equally or get serious any time soon if at all, and that it's likely the various relationships involved will evolve at different rates. Offer her helpful things to read, like this list: http://www.morethantwo.com/coupledating.html
. Tell her that all you ask is honesty, communication, and consideration and that she's under no obligations to you and/or him beyond what you all choose to agree on. But also tell her honestly that you really like her from the little you know of her so far and that you do want something serious someday, whether with her or with someone else.
You haven't mentioned Guy's feelings -- does HE want to date her? If not, you need to gently let her know that she's not dating a couple, she's dating a woman and playing with a couple. If so, then, of course, give them a chance to develop their own dynamic in addition to the you-her and you-her-him dynamic.
I don't see what your concern is about him and/or you wanting to continue to date other people? Are you afraid that a future dating partner will ask you to dump her? Are you afraid of becoming "polysaturated" amd not having enough time to seek a serious partner if you get embroiled with this woman who is probably not ready to be serious? Are you looking for polyfidelity? I would say there's no point freaking out about the future since, hey, you can't possibly predict it anyway. Just stay flexible, like you kept saying you're aiming to do.
It's going to be ok.
Congrats on meeting someone who, at the very least, it sounds like you can have a great time with!