I wonder if this kind of describing is doomed to be either too simple/narrow to fit all actual 2+1 relationships, or broad enough to make things seem more confusing than clear. As the descriptions stand, I find them a bit restrictive.
For example, there could be a non-hierarchical V, where both dyad relationships have the same priority but different amounts of time spent together due to other time constraints or different preferences of different individuals.
Or, a couple might not be living together when they meet somebody new. And even if they are, co-habitation could happen just as well by all moving together into a common apartment (or to the apartment of the newest person, though that might be less common due to space issues).
I quite cringe at the idea that a poly relationship is automatically hierarchical if some aspects, that are pre-defined as most important, are not equal/similar. I've had to question this kind of thinking a lot to reject it (e.g. I don't need to spend excactly the same amount of time with each partner, or live with both, etc., in order for the relationships to be equal). I think it comes from a cultural mindset which cannot understand non-monogamy without looking it as a competition.
Partners with Alec and Mya