Did Fred MacMurray shoot all of his My Three Sons scenes for the entire season all at once?

Yes, He was concerned about his work load and time away from his ranch and family,

Fred played hardball with his series contract. In addition to his generous salary,

the "Sons" contract was written so that all the scenes requiring his presence to be shot first,

requiring him to work only 65 days per season on the show

(the contract was reportedly used as an example by Dean Martin when negotiating

the wildly generous terms contained in his later variety show contract).

This requirement meant the series actors had to work with stand-ins and posed wardrobe continuity

issues.