Ichi Ni no San!

From hazukashiikedo:

Since middle school Bun-chan has been hanging out with the male/female childhood friend duo that is Choco and Suzu. And he's jealous of their closeness—partly because he wishes he could just generally be as close to them as they are to each other, but he has another reason: he's in love with Choco, and it seems like just a matter of time before Choco and Suzu start dating. Choco constantly denies it, though–Suzu is like his adorable twin sister. Nonetheless, Bun has never bothered getting his hopes up. And even if Suzu's adorableness wasn't a part of the equation, it's not like Bun could ever tell Choco that he accidentally happened to fall for him one day when the boob-loving Choco jokingly groped his chest and Bun got aroused by it. So Bun is mostly happy with just being friends—him, Choco and Suzu forever! Or at least, that's what he'd intended…