I think that your point in posting this is valid, there is a certain structure to most poly relationships involving three people. But the egalitarian vee or the serendipitous triad at least don't quite fit the description of 2 +1 as there is no unit standing opposed to a single person, which the way of describing it as 2 +1 kind of suggests. Maybe it would make more sense to call this three person poly relationship structures or something like that. But your general description of the structures does portrait the basic reality of most relationships we seem to come across here.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.