Most of us are familiar with the relationship format you are discussing. The concept of a couple looking to add a bisexual girl to their relationship is referred to as "Unicorn Hunting," which is why the word "unicorn" has been mentioned. Many who have experience with this sort of relationship find it to be extremely problematic in practice, and that believing it can work is nothing but a fantasy.
I have not had any experience with this type of relationship, so I can't speculate as to the accuracy of that view. I'm sure it works for some people. However, there once was a couple that wanted to add me to their relationship and it didn't work out, but I think that had more to do with the fact that I'm neither female nor bisexual than it did with the format in itself. Anyway, best of luck to you.