Who is the only actor to ever have been in three sitcoms at once?

Answer:

Frank Cady appeared as Sam Drucker on "Petticoat Junction", "Green Acres", and "The Beverly Hillbillies" simultaneously! No other actor has ever done that on three sitcoms at the same time!

"Green Acres" was a spin off of "Petticoat Junction"

The acting canine who played "Dog" on "Petticoat Junction" became more famous in the movie role of "Benji". His real name was Higgins.