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According to Varig the CV-240 did have a third cockpit crewmember: the radio operator. (https://www.varig-airlines.com/en/convair240.htm) Maybe someone else knows if this position was retained or eliminated on the -340/-440 variants?
As for the Viscount, perhaps the 3rd crewmember is 'required' in AWS more as a balance choice than a normal historical position, but there is precedent for a third crewmember acting as Navigator/LORAN operator: http://propspistonsandoldairliners.blogspot.com/2009/12/flying-vickers-viscount-across-atlantic.html
I don't think any airliners ever had 3 pilots ;)
It's about cockpit crew which includes flight engineers, navigators and radio operators. Attached pic shows a third position in the Viscount cockpit.
Cheers!
Mike
Hello
Up until the 1980s it was completely normal for a flight engineer to sit in the cockpit. This was gradually replaced by computers in newer machines. Partly to this day there is also the crew member Navigator in the cockpit. Personally, I see the staff more as a pool. The planes fly 7 days a week. When you need two pilots in the plane to fly. More pilots are needed so that the machine can fly 7 days. Pilots also have a day off and a limited number of hours per week.
greeting
Andreas