There is no "Scooby Doo 3" movie. There are two live-action Scooby Doo movies:
* Scooby-Doo (2002)
* Scooby-Doo 2: Monsters Unleashed (2004)
While there were many other Scooby-Doo projects, such as TV shows, specials, and direct-to-video movies, a third live-action film has never been released.