i think this one’s the best poly relationship where they all actually end up dating http://www.mangago.me/read-manga/i_wanna_die_before_i_m_30/
the side story in canis features a polyamorous relationship.
prologue chapter: http://www.mangago.me/read-manga/canis/mh/v02/c011.4/
first chapter: http://www.mangago.me/read-manga/canis/mh/5bab950ca56ecf9f/Ch19/pg-1/

Although it'll be hard... I'd like to ask for recommendations of BL portraying polyamorous couples / partners / lovers. I'm unsure wether there even are? And no, please no cheating or third-wheeling but actual relationships... threesomes are alright if all involved are okay with what's happening too I guess ╮( ̄▽ ̄)╭
Also if there are in other genres, Josei for example, I don't mind either! I wondered if there would be any in BL but it's likely there's some in Josei or else.