Just why was the Trident a three pilot flight deck when surely most short haul aircraft of the era had become two crew operations?
The two pilot crew aircraft up to the early sixties were propeller driven types (piston or turboprop) and hence much slower and operated at lower altitude and so were less complex. There were no two crew civil transport jet types at that time as far as I recall. Initial 737 development started about 1964 with delivery '68 I think, the first DC9 was delivered in 1965. Crew complement and associated costs did not become a major issue until the 70s.