Also the fact that a third engine might be handy for e.g. the 370-500 seat segment is clear IMO. Housing that third engine in a smart way gives various design options as we discussed some time before.
120 klbs seems a kind of practical limit for big turbofans. A bigger engine would be hard to break even for RR or GE, they already have an issue on the GE90. Apart from that it seems I've heard of a bit more then statiscally comfortable big twin incidents lately.
It is the limit for engines with bypass ratio of GE90-115, perhaps.
If you want to increase bypass ratio further, but cannot increase engine diametre, you would lose thrust.
So, an Airbus 360 replacement for A340-500/600? With 3 engines and XWB cross-section?
Or something else? Can A318 compete with the fuel burn of Bombardier C110 geared turbofans? If they cannot, and A320-sized geared turbofans are too big to be accommodated under A320 wing, what about a narrowbody trijet, with A321+ sized fuselage (like 757)?