Recently started hanging out a lot with two former female roommates. We were out and about in the city and then finally ended up drunk at a bar at the end of the night. We were playing 'Never Have I Ever' and started answering a lot of sex questions. We had somewhat established from our convo that we were open to threesomes in general. One of the girls (who I also kinda liked but recently got a boyfriend) mentioned that she would have definitely been into it with friends who were on the same page if she wasn't in her current monogamy phase. A little while after this our other friend had to go to the bathroom and she suddenly asked me if I ever thought of dating our other friend. I gave her a vague open answer basically saying 'never say never'. And then proceeded to ask her if she ever thought of putting herself in that position instead of our friend. She seemed to retort by saying no and that she had a boyfriend but she basically said no again when I asked what if before she started seeing him.

I really want to understand what's going on in her mind - would she have started/continued such a prolonged sex talk if she didn't have anything in mind? How do I get this to turn into a threesome?