While I understand that 3-engines on a 747 is perfectly safe for extended flight. What I am querying is why this is considered safer than 3-engines on a DC-10 or L-1011. With the two 3-holers, you can lose one and still be quite safe (although likely not as safe as losing one on a 747), but if you are on a 3-engine 747 and lose an additional one so you are down to two, my understanding is that you are likely in an extremely difficult situation -- and that depending on terrain or possible distance from a suitable airfield, possibly catastrophic.
While certainly the possibility of this occurring is rare, in my earlier post I just wanted to note that because of possible linkages between the ill-health of one engine and the likely health of the others (e.g., maintenance error in oil plugs - EAL, ice blocking fuel pumps - BA, fan blade failure taking out adjacent engine QF, etc.), the conditional probability that another engine will fail given that one has already failed is much larger than the unconditional probability that any engine will fail.
Please note that I am not disputing that the 747 may have additional features (e.g., 4 hydraulic systems rather than 3, etc.) that make it absolutely more secure. I am only wondering about whether given that one of its significant systems has failed, it remains more secure than a jetliner designed with this reduced number of systems -- but which has all of them operating.