That sucks but honestly just do your best to move on, let them be happy but more importantly just let yourself be happy.
Life is to short for you to let it affect you. Plays some games drink some beers then pick yourself up and find someone new who might be better and better for you than you ever had before.
Worse case scenario - they are happy and hey it is what it is
Best case scenario - threesome
Edit: Yall gonna end up making OP crazy and next thread is gonna be about how OP just killed his best friend, we have no clue how long they were together if the other friend had feelings he mentioned first, we can't say they were together behind his back. Its not good to assume. Listen OP we can't change people only ourselves if you feel betrayed in any way just get yourself into a position to move out and out of their lives, again unless there's a threesome.