Donnie? Wouldn't it be slightly hard to have a lesbian
threesome with a guy in the mix?
Anyway, this is actually a problem for many people. Some people do not even realize it because their partner never tells them. If nothing else, feel lucky that your girlfriend is open enough and trusts you enough to tell you about her girlfriend. Though your main goal is to let her know that you want to be the love of her life, not just a side-project. You shouldn't use the word cheating, as that's a serious accusation in relationships. Just sway around that and keep the focus that you want to be her friend, her comfort, her love. Do not let her try to get off topic; look her in the eyes with a sincere face while you're talking to her. If she (implies that she)
wants to be held, then hold her, but just don't break the mood. If she listens to you and thinks about it, then she is having a hard time choosing (and contrary to popular belief, that's a good thing). Let some time pass - at least a day but no more than two. While around her, just smoothly bring up a conversation about you and her girlfriend, but don't push it as far as you did before - just see if she will stay with you. If so, be glad, fill every cell in your body with joy, and express it all with your face... If not, you should let her know that you like her (or love her) so much, you'll respect her decision, (if you can live in this kind of a relationship) and you'll find a way to make it work or (if you cannot handle her having you and some girl) you can just be friends for now on. That decision is completely up to you, though at that point in time, if she has made up her mind, then believe it.
I hope everything goes well, Will! Good luck!