ahh I was searching for a story with a poly relationship for so long so I'm really happy!!...

here are two I just remembered:
http://www.mangago.zone/read-manga/ai_to_makoto_moriyo/

http://www.mangago.zone/read-manga/there_s_no_way_this_is_fate/

But if you want to search further, just take a look at this tag:
http://www.mangago.zone/tag/threesome/

though the tag does not always imply that love is involved, but you can pick from those

thank you!!