Let her know she is more important than a bisexual threesome. How you deal with her rejection is very important. If you get angry or even threaten to break up with her after she rejects your request, it will only make things worse. Obviously, your actions may remind your girlfriend that the 3some is more important in your heart than she is, even if you're just doing it to make her change her mind. You just have to spend more time to find out why she says no to threesomes and then you will know how to deal with it with more patience. After all, you love your girlfriend and you wish she would be happy.
Never try to push it or just force her to do it. No matter how eager you are to a threefun experience, you should give your girlfriend enough time so that she will think about it carefully. If you keep pushing her to make up her mind, it will only backfire. You may never have a chance for a successful threesome until your girlfriend does it willingly. After patiently waiting for the reward, you will feel that all the waiting is worth it. If you try to coerce your partner into submission, you'll regret it. Not only will you let your girlfriend down on you, you'll ruin your relationship or even break up. Only a stupid person would do that, and you certainly don't think you are.
At last, tell her everything is under her control. Sometimes, women are reluctant to have a threesome because they don't trust their boyfriends or husbands. So make it clear to your girlfriend that everything is under her control. On the one hand, she has the right to decide who will be your partner and where to make the first threesome dating. On the other hand, she can stop the dating if she finds something making her unhappy or you pay too much attention to the third woman. And promise her you will keep your words.