A triangle is the most stable structure.
A circle will collapse in on itself if you poke it, and what is a 2-sided shape anyway?
Somehow they all must have known it, even if they hadn't realised that it was true for everything.
Not only that, but it was a fine example of science trumping society. After all, they'd disproved the old adage. 'Three's a crowd' indeed.
None of them was entirely how it had started, but rarely was it regretted. They all had their issues, to be sure, and between them they had enough loneliness to form a black hole of despair. Yet somehow, even when one side buckled the others held firm.
It had its downsides, of course. After all, few beds were designed with three in mind. Not that it mattered when you were all worn out after a particularly draining case and all everyone wants is sleep but stress makes them different; snappy, brooding and emotional. It's all worth it though, when the Super's gaze flicks between them, searching for a pair and unable to see the truth. The sex is good too - far more combinations, so it's never dull. Comfortable, maybe, but never dull.
None of them understands why it works but they're all exceedingly glad that it does, because now they're stable.