here's the threesomes that i like http://www.mangago.me/home/mangalist/84846/
Ill try koisuru Arigatou
OMG. Arigatou gozaimasu
Welcome to the club! I'm always looking for manga series with threesomes and I'm so picky about the art style that it can get difficult. Leaving that aside, here are some of my personal favorites that I think you should check out. Sorry for not linking them I got a bit lazy!
Docchi no suru
Uchi wa futago de teippai
Futari no musko ni nerawarete imasu
Ryoute no darling
Junk boys
Sex therapist ( chapter 6 is 3p but you should definitely read the whole thing it's so freaking hawt!)
Kohitsuki Hokaku keikaku
Fudanshi Kun no honey days
Like they totally love each other.
I so fckin Love Chocolate Vanilla Strawberry.