I read somewhere (I think that is in this forum) that the Douglas DC-10 was created at the beginning with only 2 engines but theirs clients (AA I think...) resquested them to made the plane able to take-off from existing runways, so Douglas needed to modify the 10 desing and it was less difficult to plave the third engine in the tail than to redesign the beck of the plane.
This is the evolution of the DC-10 in an A-300 competitor, Douglas planned to release it back in '70. Sorry, forgot the web site where I picked out this picture...
Lockheed builded the TriStar with 3 engine at the startup so they didn't need to modify anything.
It's also the way that 727 and Trident were designed (with an engine in the fin, like the TriStar), so that design was prooved to be efficient.
Hpe that this can help