No one can tell you how to do this. It’s all situational. The best anyone can do is tell you how they got into a threesome.
My first serious girlfriend (actually, the person I lost my virginity to) and I were spending the summer in a basement with three other women. We were looking for work for the summer. Her best friend was in the room next door. She was supposedly bi, and I thought she was really hot. I asked my girlfriend if it would be ok to fuck her friend (the friend and I must have agreed we wanted to) and my girlfriend agreed.
After that, I was interested in a threesome, and that seemed pretty easy. I’d slept with both of them singly, and the bi friend was interested in my lover, so we arranged it. It was not as good as I thought it would be.
Later, I lived in a group house with a number of women I had slept with occasionally. Sometimes we’d be lying around, maybe after a party, just in a kind of group snuggle or something, and stuff would start happening.
There was another instance, where two of the women and I had plans for once my girlfriend left on her trip. The second she was out the door, we were on our way upstairs.
I guess in all of those cases, I had already slept with all the women involved. It was therefore easy to ask. I wasn’t afraid of rejection, since I hadn’t been rejected before. They were all somewhat relaxed about sex, so there wasn’t much of a big deal. I have to say that it’s a lot better when the two women are into you and not each other, than when the two are into each other and you—not so much. So I’m not sure a bi woman is such a good idea.
I wouldn’t want to ask someone cold to spend the night with us. They would have to be a friend, preferably a lover. This is a serious thing, not just a fuck. In my case, it was about friendship, not just sex. It was a game we played. If you advertise and go to swingers clubs or whatever, then it’s pretty sterile. It won’t be much fun. You might as well hire a hooker. It’s a lot more fun if everyone is into it and you all like each other—a lot.