The real advantage of three engines is, in the event of OEI, one is not required to land, one can continue, if the performance allows. That is, you have the MEA accounted for. In the end, once an engine dies, a twin is now a single-engine plane not going far. ETOPS is a consideration for everyone who wants to run a airline-grade operation.
The BA 747 is an example of four-engine capability.
GF