Somegeezer pretty much summed it up in the statement, 'There are as many ways ...'. There are so many variations available so you can customize your relationship to your needs. What I can tell you as a former poly triad to triad vee partner, one of the most difficult components to this relationship is finding one person that you both want to develop more with. It seems to me that you are seeking a female partner who is involved with both you and your boyfriend sexually (but only as a couple) and, moreso, you emotionally? Speaking as a unicorn, adding a single female to the 'family' but only allowing interactions as a 'package deal' may be a bit difficult, but not impossible, to organize for a satisfying, closed poly triad for all parties involved.
I look forward to watching your journey.