I only know of these two, they're both GL though:
https://www.mangago.zone/read-manga/luminous_blue/
https://www.mangago.zone/read-manga/goukaku_no_tame_no_yasashii_sankaku_kankei_nyuumon/ (age gap? don't remember)
Wish i knew of more but this is all i have, both BLs. I usually have to read books for better poly relationships since its not a popular theme in manhwa/manga/manhua
In the Castle: the Dragon's Erotic Education (Yaoi)
https://www.mangago.zone/read-manga/in_the_castle_the_dragon_s_erotic_education/
Who is a sweet cheater? (Yaoi)
https://www.mangago.zone/read-manga/who_is_a_sweet_cheater/
finished both and they're just straight up porn
omgg tyy
LMAO i'm dead at the fact that you've already read both. But no seriously! it's all smut when it comes to poly on here :(
it hardly even pass as poly
100% agreed, it's more like the Semes just settled on "sharing" but it's not a real relationship on all ends
anyone who knows poly story? like actually poly idc if it's bl or nawt, like don't recommend me lick me, love me cause that shit isn't poly