Some people think that a triad wold be ideal, because they think (before they have experience) that they will somehow be less likely to feel jealousy if everyone is involved together.
But some people like the triad (or quad, etc.) connection because it creates a sense of family. (There's me and him, and me and her, and her and him, and then there's US, which is a separate and excellent dynamic all its own.)
That's been my experience, at least. I stumbled into it, though, and can't imagine how someone would go about creating it intentionally.
I think it is possible, though, to be a unicorn who is attracted to couples. I haven't yet decided exactly how I feel about that, or what the driving force seems to be that underlies the attraction.