Hi there Scott - welcome aboard.

This phrase was definitely in use before films - or even photography - had been invented. I suspect that it might even be pre-Victorian, although there is no doubt that it was in use then - with the meaning given above -"Every Tom, Dick and Harry will be there."

The three names were probably chosen for a) the frequency that they were used as names, and b) the euphonius sound - almost poetic, as Dylan Thomas realised when he utilized them for Polly Garter's song (apart from the connotation that every T,D & H went with Polly)