Figure[White to move]

The trapped Black king and the White battery on the e-file suggest a back rank mate, but e8 is guarded twice; so White uses a piece from outside the battery to take out one of the guards: 1. RxR+, RxR; 2. Qe8+, RxQ; 3. RxR#. It’s a simple combination of the back rank mating pattern with a removal of the guard.