These three are the only theater mass shooters I'm aware of, but there's two more that may or may not count as theater shooters, too, depending on your definition: Seth Ator, who was shot dead in the parking lot of a cinema, and Joseph Jimenez, who committed a double homicide that fits every parameter of a mass shooting besides the death toll.
If you count bombers, Gao Haiping was the most successful theater mass murderer.