Three engines gave the long-range Iris a margin against forced descent at sea. It could maintain height at full load on any two. Losing one engine therefore left near-normal flight available. After losing a second, it could still keep going, although it would probably fly lower than desired.
Notable Point · 0:29-0:48
Three engines for a safer overwater patrol
The Iris used three engines to reduce the risk of a forced descent at sea. It could maintain height at full load after one engine failure and continue after losing two, though probably below the preferred altitude.