I get these conversations out of the way as soon as i meet a person that i really like.Just ask her has she ever tried it before even though you know that she has not.See how she reacts like if she seems to like the idea or if it makes her look disgusted then just talk about it with her and you will get an idea if she is interested in that kind of thing.You never know it may be something that she has always wanted to try.There are plenty of ways you could get into the subject of 3somes.