Triple-Cross

Backstabbing through entrapment

luring a friend into being crossed is to double-cross/betray, then actually crossing them is a triple cross.

Billy shot a stranger - he crossed him
Bobby shot a friend - he double-crossed him
Quaddy said he wanted to meet up with his old 'friend' and shot him - he triple-crossed him

A successful undercover cop is a double-crosser.
An undercover cop who encourages crime / entraps - is a triple-crosser