Imaginext: Fortress of the Dragon (2003)

Meet Marty, Dan and Katy--three ordinary kids who find themselves in the middle of an extraordinary adventure when they discover "The Books of Imaginext". A rare, one-of-a-kind secret comic book series that magically transports them into the world they're reading about. This time, the trio venture to a medieval kingdom to help King Bravemore battle the dastardly Lord Dragonmont and his powerful dragon.