My Fiancé and I are interested in a FFM threesome with a girl we've been interested in.

My Fiancé and I are interested in a FFM threesome with a girl we've been interested in. She's very interesting in us both and she's agreed to do it with us. I'm nervous as well as excited. Even if I'm a virgin. I know what to do in a threesome. But I'm just really shy about it.