The single opening for a frogs digestive, urinary, and reproductive system is called a?

It's called a cloaca. It serves as the anus most commonly in birds, fish and amphibians (including frogs), however unlike reptiles and mammals, the cloaca also serves as the urinary tract and the reproductive tract, all in one. As a reproductive tract, the cloaca allows for the passage of sperm from male frogs and eggs from female frogs.