They were always going to go 3-3-3 for the A350. The fuselage width I would say is in between an A330/A340 and a 777. Think of it that way. 3-4-3 on the A350 would most likely be for ULCC's. IIRC, AirAsiaX will be the first 3-4-3 operator for the A350.
As for 3-4-3 on the 777, it wouldn't surprise me if they do go that way. 3-4-3 will be standard on the 777X.