I honestly find that being in a two-person relationship is enough to manage. If we added a third person, I think we'd kill each other.
I am oddly fascinated by a couple-cum-throuple I knew from a few years ago. Originally a very narcissistic American and his British husband. The American clearly being the dom top in the couple. Another American was added last year, and the British husband looks incredibly unhappy and dead-behind-the-eyes in all their photos and Instagram stories, while the two Americans seem very happy. I do wonder whether I'm applying my own prejudice onto them when I look at them, but knowing what I know about their personalities, it tracks that the British husband has given in to his American husband's demands, rather than wanting this in the first place.