I suggest you follow Neon's links and see what you find. Triads are well talked about here and I am sure that you will find some useful and interesting threads if you do a tag search.
The relationship dynamic I am in is a vee by sex only. This seems to be the case with most established vees. There is often not much future in a vee where the parties are not equal in terms of status. Not to say that longevity is the goal, but it quite often is.
In my experience a metamour relationship between all parties is necessary in order to sustain a relationship. My PN and Mono are very good friends and are closer to each other in their lives than anyone else besides me. They really are the back bone to the success of our relationship. If they had not ended up being close friends I would not be with either of them today. I fully believe that. I rely on them to keep their friendship together in order for "us" to succeed.
Some people seem to think that in order to be a triad that everyone should be having sex. That is not necessarily worth pursuing I don't think. Some people think that if you are in a vee then the two arms are not close and are completely separated from one another; again, not necessarily the case. Most long standing vees make me think of a "hug" more than a vee. I am the body, they are the arms crossed over in a such a way as we are all giving ourselves a hug.
Have you seen the book, "the hug?" Its a kids book with a cover that makes me think of vees. Great book btw.