I've only seen a few reruns of "Combat" but I'm aware the series ran for many years, in the sixties, and depicted situations ranging from the battlefield to behind the lines espionage. It is safe to assume the studio rented every privately owned World War Two AFV available.
According to Wikipedia the M3A3 appeared, as noted, appeared during the fifth season of the Twilight Zone", "The 7th is Made up of Phantoms", and on "The Green Hornet" "Seek, Stalk and Distroy" episode originally aired on January 6, 1967. They seem to have missed the "Batman" episode which is odd since the sight of the Penguin, in a purple hat, riding on a gold Stuart tank, is a bit hard to miss.
The "Twilight Zone" note withstanding the M3A3 was never used by the National Guard or any other branch of the US military. It was produced to be exported under the lend lease program. The few examples, still state side, after the end of World War Two, were put on static display in parks and war memorials around the country. While several restored M3A3 exist in Europe the only running example in the US, possibly the the one used in all those sixties TV series, is now part of The Planes of Fame AFV collection in Chino, California.