My boyfriend and I didn't set out to have a mutual girlfriend. Maybe that's why it worked. In fact when my boyfriend and I started dating I was just getting out of a relationship with a couple that ended really badly. I felt awful about the idea of someone dating me to be with my boyfriend the way I felt like I had done in the past (thankfully short) relationship.
We where open and he started dating someone that I liked and I worked with. She slowly became a really deep friend and we were at times sexual. Then about two years into my boyfriend and her dating we decided that we where in fact partners and we are working on becoming a balanced open triad.
It wasn't what I was looking for. It wasn't what I would have even asked for but it makes me so happy. It took years and we still aren't fully balanced (hopefully that will change when we all move in together this summer) I think it is ridiculous to set out with a relationship that looks like ours as a goal. I think having a goal as to what your relationship "should" look like almost always screws you up in the long run. We just went into the world with our hearts open and willing to push ourselves and try to make things work and this is how it turned out. But we are open so who knows what it will look like in six months - 5 years. I hope there are more people we love and share our lives with but I don't have an ideal shape for our relationships just the goal of always being open to where life take you.