Yes, I think a triad would be the most likely configuration, but it would hopefully not be "a 3-person ordeal" as you characterize it!
For you and most others, exclusivity may be a crucial element of intimate relationships. But for some of us, sharing can be extremely powerful and satisfying.
By "3-person ordeal", I merely meant to refer to a 3-person relationship without using something loaded like "affair" but still implying the complexity that would necessarily be built into it.
Perhaps I should rephrase: what I meant is that it isn't healthy for there to be inequalities in a relationship. And I suggested that it would be difficult for a plural relationship to exist without inequalities. You mention that "sharing" is powerful and satisfying -- do you mean that it is equal sharing? This is the topic that I was trying to address. And naturally, I would agree that it is the part most people would have a problem doing.
Also, I don't think "exclusivity" is really the right descriptor -- it immediately suggests a possessive view to relationships. Perhaps "commitment"? This shouldn't be taken to demean anything, but I think it has a more positive "giving" connotation than the negative "sacrificing" connotation that exclusivity has.
I think that there are almost always inequities in relationships. My wife gets something somewhat different out of our relationship than I do. There must of course be reciprocity, love and caring in both directions, mutual respect and support, but "equality" does not really capture the essence of most relationships.
In a threesome, I do not see equality as the primary goal. All three relationships must be strong, but concern about everyone being on exactly the same footing could be destructive. Taking pleasure in the relationship that the other two have is extremely important.
And there is the three-way relationship, which ideally integrates and overarches the three pairwise ones.
When I was in a MMF menage for three years, there was never an issue of: Does she love both of us equally? All three relationships had their own unique qualities, and all three were intensely sexual. Did we worry about such things as: Can she cum more easily with one of us penetrating her than with the other? Not at all!
I think the term "exclusivity" and even "possessiveness" is in fact rather descriptive of most long-term relationships. I definitely do not equate "commitment" with limiting sexual activity to one person. Deep, long-term relationships are indeed possible without sexual exclusiveness. In fact, some relationships are stifled or even destroyed because of an unrealistically narrow view of "commitment".
Certainly, twosomes are the only way to go for many people. But as society gets increasingly tolerant of diversity, and as many people get more and more in touch with their own bisexuality, it seems likely to me that the day is coming when threesomes and other types of plural relationships will become no bigger deal than are gay and lesbian couples today.