Oh dear now you are talking about really bringing the triangle to love triangles fr. Tho I can think of two I have yet to read one where the endgame is actually a poly relationship, so my recs are limited to ending with only one (endgame being BL since that's what you are searching at the end of the day)
• Ao no flag (Manga, fluff and angst tbh, slice of life)
[Top tier, realistic and wholesome af, you won't regret I am being serious]
https://www.mangago.me/read-manga/ao_no_flag/
• Faded first love (Manga, comedy, slice of life)
[This just way too funny and it always makes my day. The author is also like one of the best so i truly recommend]
https://www.mangago.me/read-manga/faded_first_love/
kind of except it's character (a) likes character (b), but (b) likes (c) while (c) likes (a) and character (d) also likes character (a)
https://www.mangago.me/read-manga/spring_is_near/
Thank you for that reco but I actually have already read those IT WAS AMAZING
Well I better prepare myself for some more angst
RECOMMEND A BL REAL LOVE TRIANGLE PLS!! And no, I'm not talking about character (b) and (c) fighting over character (a). I'm talking about character (a) likes character (b), but (b) likes (c) while (c) likes (a). BONUS if they end up sème x seke x uke poly relationship