Jk here
http://www.mangago.me/read-manga/groupie/
http://www.mangago.me/read-manga/satou_kun_ni_wa_kanawanai_sachimo/
http://www.mangago.me/read-manga/i_wanna_die_before_i_m_30/
http://www.mangago.me/read-manga/futari_no_musuko_ni_nerawarete_imasu/
But Idk about hidden gems cuz you can literally look them up by using threesome tag
http://www.mangago.me/read-manga/seikaku_warui_uke_ga_guzuguzuni_nakasareru_hon/
Chapter 11 is a gem you must read it!
Any good, completed and hidden gem threesome suggestions?