So I really get annoyed when people keep recommending the same big-name authors for this niche. If you want that authentic, dragged-out tension where everyone's dancing around each other for ages, the stuff on mainstream storefronts often cuts it too short to pay the bills.
I've had way better luck digging through dedicated forums for polyam romance. There's a subreddit, I think it's r/RomanceBooks, where if you use the right search terms you can unearth some self-published gold. Authors like Lola Malone and J.A. Low have a few titles that stretch the mutual pining over several books, but even they sometimes rush the final get-together. The real slow-crawlers seem to be on sites like Literotica, buried in the 'Novels and Novellas' section—you just have to filter through a lot of, uh, less plot-focused stuff. The buildup there feels more organic, like the characters are actually terrified of messing up the friend group.
Honestly, half the appeal for me is the anxiety of whether the third person will even be welcomed, and you need serious page count to sell that.