The first page especially has a lot of good ones: http://www.mangago.me/tag/threesome
Here are some hand picked ones:
http://www.mangago.me/read-manga/bokura_no_mitsudomoe_sensou/
http://www.mangago.me/read-manga/sefure_jijou_koibito_miman/
(We are still waiting for the threesome to happen in the next one:) http://www.mangago.me/read-manga/oboreru_triangle/
Why stop at threesomes?
http://www.mangago.me/read-manga/yarichin_bitch_club/
(This one is my personal favorite:)
http://www.mangago.me/read-manga/bokura_to_usagi_no_koi_wa_sensou/
My favorite threesomes. Not all love triangles though. http://www.mangago.me/home/mangalist/116326/
Lol. Or just good angsty love triangles in general.
I already know choco strawberry vanilla and sankokou opera.