Continuting on 3
Just to address one of the many aspects of this increasingly convoluted debate:
The problem of a second engine failure has been raised by several contributors to this and other threads. I'd like to point out that we wouldn't sit there in three-engine cruise with our fingers crossed, repeating "I hope I hope I hope another one doesn't go".
Sadly, the general public is often given the impression that this is how passenger aircraft are flown on a regular basis.
In this kind of flying, it is essential to keep a running game plan that covers critical failures. Where would you go right now if a passenger suffered a heart attack? If there was an uncontrolled fire onboard? If an engine failed? It's like defensive driving in a way - you don't just wait for a problem to hit you, you prepare for it in case it happens.
If you should find yourself flying a 747 on three engines, you MUST have a game plan that includes a subsequent failure on the same side, however unlikely. This isn't just a bit of theory - it's what pilots really do.
I only mention this because someone asked something like, "what if you were over high ground and had a second failure?".
You wouldn't be there, because you'd have looked at your two-engine performance and decided the Himalayas might be a fine place for Michael Palin, but not for a two-engined jumbo. And therefore you wouldn't fly over them on three.
Smudge