i humbly present nicorinpana...
nicorinpana, eh... good one. I like it. alright, I'll tell you what I think it looks like, and how I think it happened, as promised. (note from zin further down the post: this got out of hand and is closer to a fanfic than just an analysis. if someone wants to turn this into an actual fanfic they're free to but they have to tag me when it's done)
I feel like everyone's rooting for rinpana. they're so cute together, they promised to marry each other some day, it's all very sweet. and I think nico's jealous of that.
I think hanayo probably went to her for advice on how to keep things secret when she actually confesses (rin assumes they're already partners, so she doesn't see a need to confess, and hanayo's too nervous to just Do things without being sure of how everything will go), and I think nico clenched her fist and did that grumbly twitchy expression you see on her sometimes. she did give advice though, mostly on what not to do, based on gossip she's read and heard about other idols. and I think hanayo thanking nico for this advice was adorable.
nico's in denial for a while. she watches them be all clingy and lovey-dovey and she looks away. it's improper, after all! honestly... but she doesn't think she can achieve something like that. well, I mean, someone as cute as her could get any person she wants! but... it's not really about that, is it? but then what is it about? she's not envious of just one of them, so she can't be a homewrecker. it's the dynamic she likes, and her brain keeps trying to insert herself into it. but... that doesn't make sense, does it?
rin notices nico looking, and she notices her looking away. she doesn't really get it, maybe, but she points it out. hanayo starts seeing it too, and she's eventually given the courage to talk to nico about it. rin would, but she's not very subtle, and this is delicate.
rin's outside when they talk, thinking. if nico was mad at them, would they have to break up until they stopped being idols? of course they would only marry when they were grownups anyway, and they're best buds as it is, so it's not really a big deal. marriage, though... rin remembers love wing bell, when several of them were dressed up as her bridegrooms. kayo-chin was the focus, but it would be kinda nice to have more than just one, it was fun! was nico jealous because she wanted to be rin's bride too? that would be sad... sadder still if kayo-chin wasn't okay with it. nico was a bit of a meanie sometimes, but rin did like her, and rin reckoned it'd be fun to kiss her. if kayo-chin was okay with it, of course. which she might not be. hmmm...
does nico think they should split? is hanayo out of her MIND? of course not! and the conversation takes a while, because it's hard for nico to be honest with herself, but eventually hanayo figures it out. she blushes to her ears when she gets there. nico hasn't pieced it together yet, and urges her to just talk. kinda hard to do when you're a blushing mess, but she does stammer out that may-maybe they c-could ask ri-rin-chan abou-about all being gi-girlfriends t-t-together? nico goes through a very fun face journey and eventually says fine, if that's what they want so badly, perhaps nico will consider it. but if they ruin her reputation they'll be dropped instantly, okay?
nico's the guide of the conversation with, hanayo can barely nod along she's so embarrassed and scared. rin doesn't think it's a problem at all, and she tackles the both of them in a hug once she's got confirmation, immediately saying she loves them both.
imo each of them is romantically involved with both of the other two. I don't think the triad is fully closed, but I don't think they're actively dating outside of each other for the moment either.