Name an OTP that fits this description! >_<
Eren x Mikasa - Attack on Titan
A = Mikasa
B = Eren
C = Armin
Bakugo x Kirishima - My Hero Academia
A = Kirishima
B = Bakugo
C = Kaminari
Izaya x Shizuo - Durarara!!
A = Izaya
B = Shizuo
C = Shinra (or Celty... or both)
Naruto x Sasuke - Naruto
A = Naruto
B = Sasuke
C = Sai (lots of possibilities, but I like his bluntness!)
... I probably have more, including some non-anime examples (Supernatural's Destiel, C = Sam; Harry Potter's Drarry, C = Snape, I think, also generic Slytherpuff ship, C = Ravenclaw, probably; maybe Doctor x Missy from Doctor Who, C = Nardole; basically any yaoi trash ship), but just to name a few for starters!
Add more in the comments! This should be fun!