So my best friend is polyamorous, I’m monogamous, and (I’m 99.9% sure) my girlfriend is monogamous. (We’re all girls by the way) My best friend Just told me that she has a crush on my and my girlfriend, we’re all best friends. I don’t know exactly how to react, I’m not gonna cut off ties because she likes us. But I’m not interested in a polyamorous relationship. I don’t think my girlfriend is either. My best friend only told me (so far) so I’m not going to tell my girlfriend yet. Cause she’s probably not ready to tell her if she hasn’t already done it. But how do I let her down slowly? I’m so confused and stressed, how long has she felt this way? How’s my girlfriend going to react? What if she *does* want an open relationship? What if my best friend isolates herself because I don’t feel the same way? I’m in shock, and I can’t believe this is real. (I don’t mean to sound rude or disrespectful in any ways, I’m just not very educated on this subject.)
There isn't a way to prevent someone from feeling disappointed or even hurt by you rejecting them. And the only other alternatives are completely ghosting her or going on with a relationship that you don't want. You just have to be honest and say something like, "I appreciate that you felt comfortable telling me this. As you know, I'm monogamous. I don't have any interest in a polyamorous relationship and I don't feel that way about you. I'd like for us to continue to be friends."
If she has become your best friend because she feels romantic toward you, she may not be so close with you after you tell her this. Unfortunately, there isn't very much you can do about that. At the end of the day, your best friend started to have feelings for yourself and your girlfriend and decided to continue having those feelings and putting herself in situations where those feelings might continue most likely knowing you are monogamous. Had I given advice to her about this situation, I would have told her not to expect that a big confession would help the situation and to maybe ask where you stood on polyamory and, if you said you weren't interested, to drop it.
If you don't want polyamory, do not try it for the sake of trying to keep your best friend in your life. If your girlfriend is interested in dating her, unfortunately this might also mean ending your relationship with your girlfriend if she definitely does want to be polyamorous. I don't mean to make light of how serious that is and it would obviously be upsetting to you, but I feel like if the alternative is pushing yourself to be polyamorous and you have no interest in doing so... then that is a far more painful alternative than it might seem right now.
So, to sum up, I think you should just tell her, not slowly, but up front and honestly. You cannot completely control her emotions or feelings. Obviously, don't be a jerk about it, but being honest and straightforward about your feelings is important to do regardless of what relationship style you have. Speak to your girlfriend about it. I wouldn't wait for your friend to tell her. If your girlfriend does want to try polyamory, make it clear this isn't what you want. I'm sorry that this had to happen this way, however. I think that suddenly telling you this isn't really helpful. You don't have to cut off ties with her, but if she doesn't respect your wishes and wants, then it might be best to regardless of how she does her relationship.
I hope this helps and good luck!