Last anon gave me an idea...
Breezepelt has been seeing both Lionblaze and Jayfeather off-and-on. It's complicated, because he didn't get along with either of them as apprentices, but trouble at home and seeing them frequently along the border and at Gatherings, stuff just sort of happened. They're both cats he can be himself around.
Lionblaze and Jayfeather eventually figure out that they've been sneaking out to meet the same cat when Lionblaze tries to "surprise" him during a time he was already with Jayfeather. It's very awkward and both relationships get rocky. Someone (maybe a fourth cat... Heathertail?) floats the idea that they could all just date at once? Like why try to manage these two relationships when you could just be a big throuple.
"That won't work, we're brothers," Lion and/or Jay insists.
So things are rocky for a while but persist in some form. Then, the big reveal happens and stuff gets wild.
A bit thereafter, Breezepelt and Jayfeather were going to meet up for the first time in a while. Lionblaze tags along.
"I thought you said we couldn't do the throuple thing?"
"Well, I guess we're all brothers now, so... fuck it."