Ouroboros
mm that's hard since I feel like it's usually more threesome, 2 guys on one guy rather than the more balanced nature in boyfriends. Here's some that apply though!
https://www.mangago.zone/read-manga/there_s_no_way_this_is_fate/
https://www.mangago.zone/read-manga/choco_strawberry_vanilla/
https://www.mangago.zone/read-manga/i_wanna_die_before_i_m_30/
https://www.mangago.zone/read-manga/oboreru_triangle/
https://www.mangago.zone/read-manga/futari_no_musuko_ni_nerawarete_imasu/
https://www.mangago.zone/read-manga/docchi_ni_suru/
https://www.mangago.zone/read-manga/a_dead_crow_s_stare/
https://www.mangago.zone/read-manga/pet_keiyaku/
https://www.mangago.zone/read-manga/bad_prince_honey_party/ <- i think this is the most poly, in terms of actually liking each other
Might want to check out these tags:
https://www.mangago.zone/tag/polyamory/
https://www.mangago.zone/tag/threesome/
yaoi with a poly relationship? kinda like “boyfriends”