Black’s king is trapped and his back rank is weak (no defenders reside on it), so White can go for mate with a single heavy piece: Rc8+. All Black can do is throw pieces into the path of White’s rook, but this time the rook just eats them up. It goes 1. Rc8+, Bd8; 2. RxB+, Re8; 3. RxR#. Black had only useless interpositions.