Funny enough, there's a oneshot where the characters are called A, B, and C. It's also got a similar plot to what you're long for. It's probably not what you're looking for, but give it a read anyway.
http://www.mangago.zone/read-manga/a_senpai/
*looking for
thank you i'll give it a try ヾ(☆▽☆)
I hope you enjoy it!
prolly not what u r looking for, but the plot is pretty similar to: http://www.mangago.zone/read-manga/kimi_no_kao_ni_sasu_kage/ A confesses to B, B panics( he likes A, but he is afraid to admit it) so he introduces A to C. A sleeps with C pretending to be B(and C knows about it) and....okay i dont want to spoiler any further. u can check it out if u like^^
so this is kind of confusing but here it goes Lets call him A, A likes B and B likes A but they are to afraid to tell there feelings for each other so A sleeps with C and pretends that it's B when they are fucking but C knows that A and B love each other and i feel like theres a piano involved somehow thank you