Ridley Pearson has said on his blog many times it's set in Epcot. At the end of the second book, it mentions the villains heading in the direction of Hollywood Studios, and we know that Epcot is joined to DHS by a lake, plus many hotels.
Here's the plot of KK3:
When Disney Imagineers installed hologram guides for the Magic Kingdom, using teenage models they had no idea the technology might backfire. But backfire it did: some nights when the kids go to sleep, they wake up in one of the Disney parks as a hologram.
With the adventures set forth in the first books now behind them, Kingdom Keepers 3: Disney In Shadow follows the five teens, Finn, Philby, Willa, Charlene, and Maybeck as they search to find Wayne, their mentor and head Imagineer who has mysteriously gone missing. Concerned Wayne has been abducted by the Overtakers-Disney villains, who along with other Disney characters, take over the parks when the turnstiles stop spinning, and want desperately to steer the parks to a far darker place-the five kids pick up a major clue from a close friend, Jez, whose dreams (nightmares, really) often accurately predict the future.
The very few clues from Jez's dream lead the kids into Disney's Hollywood Studios and Epcot--through imaginary worlds that become real, by imaginary kids who are real. Each clue seems tied to the last, and with the stakes growing ever higher, what starts out as a puzzle ends up as a fight for their lives. Through a transparent paper box, a quest for a sword, rides on Soarin' and Maelstrom, life-and-death encounters with giant snakes, and a devious Maleficent, the Kingdom Keepers not only begin to decipher deeper meanings to the clues, but discover new truths about themselves and their ever-growing friendships.
Anyone got any more info on that?? 
