My 2nd time reading this.

I think initially C-Kun liked B-Sempai more... and although both A and B had feelings for C-Kun, A-Sempai (not being scared of his sexuality) acted on his feelings first or, made the first move. Upon hearing C and A in the throes of sex he became enraged with jealousy (wishing it was him with C and not A) spread the rumors out of anger. Now, seeing A come to his rescue I believe C-Kun became totally devoted to A. I mean we did get a small glimpse of A's face. Anyway, this is just my theory so I could be interpreting this wrong.