As you write, at the time those trijets were designed they enjoyed preferential treatment over aircraft with two engines for overwater operation. They also were more efficient than comparable designs with four engines. See here for that discussion.
However, the wing-mounted engine has some advantages:
- Flutter damping when mounted ahead of the elastic line of the wing
- Bending relief
- Easier access for maintenance
- Shorter fuel lines
The tail position was chosen because three engines were enough for the optimum size of those airliners and equipping them with four engines had lowered efficiency. If you need to put three engines on an aircraft, one has to be in the line of symmetry, and here the best compromise was on or in the rear fuselage. Had more powerful and reliable engines been available back then, you can be sure they would had used only two engines and mounted them on the wing. But we had to wait two more decades until turbofan engines were powerful enough and the certification authorities were convinced of their reliability.
The fashion of early jets to put all engines at the back was just that and had ended before the trijets were designed.