Walt Disney Studios has begun early development work on a third installment of the "National Treasure" adventure franchise, according to industry insider Daniel Richtman, who has a track record of accurately revealing upcoming studio projects. Disney has not formally announced the project or set a release date, but the reported move would revive a series that last reached cinemas with 2007’s $459 million-grossing sequel "National Treasure: Book of Secrets." No cast or director attachments were disclosed.