Theodore Sturgeon's 1951 story “Rule of Three” (spoilers!) features two tripartite energy beings who embed themselves in six humans, and then try to reintegrate themselves by getting the humans to bond in two groups of three, but when that fails, they switch to three groups of two and all is fine. There's a bit of interesting gay subtext and hints at the possibility of three humans finding happiness together, but the people end up as three male/female pairs.
In 1979, Sturgeon wrote an introduction to a reprint of the story, in which he said:
Although the person who wrote “Rule of Three” clearly regarded the desirability of monogamy as axiomatic, the astute reader—another term for postgame quarterbacking—might find in it the seeds of later ideation. One tends to work out one's own convictions in writing fiction—especially in science fiction—and to test them against possibilities, however untimely or unformed or wishful or improbable. Anyway, in this story (1951) one may find what is possibly the first suggestion in science fiction that love may not after all be confined to gender or to monogamy. Here are the seeds of later work like More Than Human, and the growing concept that perhaps, after all, the greatest advance we can make is to accept what we are, and then to grok, to blesh, to meld, to join. Real science fiction talk, that, ain't it?
(As quoted in Baby Is Three: The Complete Stories of Theodore Sturgeon, Vol. VI, in the story notes on p. 406.)
I agree that this story contains intriguing seeds of later work and ideas. He wrote “Baby Is Three” about a year and a half after writing this story, and expanded that into More Than Human a year or so later. And right around that time, he wrote “The World Well Lost,” which I think is generally regarded as the first positive portrayal of a gay character in sf (see also my 2003 entry about gay characters in sf); arguably there are positive portrayals of gay or bi characters in “Rule of Three,” but it's hard to be certain through the layers of euphemism, and anyway one of them ends up dead. So it's nice that he revisited that a couple years later with a slightly more direct approach.
Anyway, mostly I just thought the quote was interesting, and thought some of y'all might also find it interesting.