12 people were killed and over 50 were injured during a midnight screening of Christopher Nolan's The Dark Knight Rises at the Cinemark-operated theatre on July 20.
Deadline reports that three of the audience members injured during the attack are now suing Cinemark with claims that the company failed to supply the Century 16 location with sufficient security.
A suit filed by victims Denise Traynom and Brandon Axelrod reads: "The gunman made one or more trips from his car through the open exterior door of Auditorium 9, bringing his arsenal and ammunition through that open door.
"Throughout that time, no employee or security personnel contacted him, deterred him, monitored him or stopped him from that re-entry."
A man named Joshua Nowlan, who received several gunshot wounds, has also filed similar complaints today (September 22).
The three victims are now seeking a jury trial, and damages exceeding $75,000 (£46,160).