It all started with the Lockheed Electra. The Pilots Union got it mandated that the aircraft would be flown with 3 pilots and not 2 pilots and a flight engineer. ALPA also wanted all jet equipment to carry a third pilot.
That's not 100% true. We (NWA) operated Electras with two pilots and a professional FE. We operated the Boeings with three pilots.
Goldfish