They only need 2 of the 3 engines to perform the landing flip and 1 of the 3 engines to perform the landing. So they do have some redundancy, and the computers were very likely programmed to light the third engine when the second failed. That this didn't happen was either due to debris from the second engine taking out the third during its failure or the failure being in a common area like the downcomers or that the third engine was still too hot -- the third engine was the one that was used to hover at apogee. Or some other reason.
