Hi, I'd like to help you with your question.
I am sorry this has happened to you. It is understandable that you want your friends to stand behind you. You were hurt and they need to understand. However, this may be asking a lot from them. They do not feel the same way about this person and therefore do not feel the same sense of betrayal and hurt.
You have a couple of options here. You can talk with your two friends about how you feel about what happened to you. Expecting them to act against this person without knowing how you feel is unfair. You should tell them that this person hurt you and that you feel uncomfortable at best being in a room with her. Then let them make the decision on how they want to handle it.
Two, you can end your friendships with your two friends. This would not be an ideal option, but you should consider it only because it is an option. However, if you chose to do this, you will lose not only one friend to betrayal, but two more in the process. The two remaining friends may also be more willing to see the other person's side of this situation.
Three, realize that the person who hurt you is to blame, no one else. Insisting others feel your pain as much as you do and react the way you would does not help you and even makes this situation worse.
Make some dates to go out with your two friends by yourselves. This can help strengthen your relationships and will guarantee that the other person is not going to be around.
When you see the other person at events, ignore her. Take another friend who is not involved with you for support. Avoid her if you chose, but try not to let it bother you. When you react to her, it gives her more power over you. That is not what you want. You are the one who is right here. Don't let her dictate how you should feel.
If you have someone who is willing to listen, talk about how you feel. You need to work out how this affected you and someone who can give you good feedback will help. Chose someone outside of the situation if you can. If not, try seeing a therapist. A therapist can help you work through your feelings. You can ask your doctor for a referral or if you attend church, try your pastor. Also, you can search on line at www.findatherapist.com.
Also, take care of yourself. Your self esteem has taken a hit from this. Try going out and treating yourself to a massage, manicure, see a movie with a good friend, or buy yourself a new outfit. Give yourself a boost somehow. It can work wonders when you feel upset.
I hope this has helped,