Try reading this one http://www.mangago.me/read-manga/ai_to_makoto_moriyo/ Ai to Makoto (Moriyo) (Yaoi) manga its omegaverse, so its have its specifics, but its quite nice read.(≧∀≦)
http://www.mangago.me/read-manga/choco_strawberry_vanilla/
I liked this one.
Or go for 30 made shinitai
I was also hoping for poly initially, so I feel your pain. I think I've got every vaguely decent threesome manga bookmarked. Unfortunately, ambiguous threesomes are common, but full on poly tends to only be implied at best. But here are some links.
I Wanna Die Before I'm 30 (already mentioned, but not linked): https://www.ebookrenta.com/renta/sc/frm/item/140197/
Yupiteru ni Onegai: http://www.mangago.me/read-manga/yupiteru_ni_onegai/
Stuck Between a Sexy Rock and a Hard Friend: https://www.ebookrenta.com/renta/sc/frm/item/138733/
Uchi wa Futago de Teppai! (kind of): https://myreadingmanga.info/kamiya-memo-uchi-wa-futago-de-teippai-eng/
And then some more that fit the bill, but which aren't faves:
Boys Love!: https://read.futekiya.com/comic/5d920d3f4c1ef
Mob Kei Danshi: http://www.mangago.me/read-manga/mob_kei_danshi_no_yuuutsu/
Bokura no Nichijou no Mouhitotsu no Saundotorakku: https://myreadingmanga.info/bokura-no-nichijou-no-mouhitotsu-no-saundotorakku-eng/
Still hoping for the day where a healthy 3p relationship dynamic gets portrayed in bl :')
I've been thinking that I usually like 2nd couples better because they aren't so dramatic. Also, it's great that sunyool gets his happy ending <3