Don't want to get your hopes up too much, but sometimes you think you know someone & then suddenly they change.... when I started dating my wife we watched some porn movies with her girlfriend which contained some FFM threesomes.
She was turned on very much, I know this because after the movie was over her friend left the room to shower (she was obviously turned on too!) she tore my clothes off & we made love on her friends bean bag!
The subject of threesomes came up over the years & she always responded in a negative way, but then suddenly it happened with our long time female friend
Maybe you could test the waters so to speak & get a threesome video for you to watch?