One out of three of them have been confirmed by the developers. Diancie was officially revealed via the official X and Y site and Pokemon's YouTube channel a few weeks ago, and fans are expecting an event for it soon. It has also be confirmed to have a major role in an upcoming Pokemon movie, so it is definitely part of the game.
Hoopa and Volcanion, however, have not been confirmed by the developers, but evidence suggests that they do indeed exist. Since the three of them were revealed at the same time by hacker Smealum, they appear to be a trio of some sort, which makes the thought of the three being event legendaries very possible. Also, it has been proven that they do indeed exist in the game's coding thanks to some tests with the GTS, which considers them valid Pokemon.
So either they will be revealed officially in the future, or they will just be left in the coding never to see use. But we do know that Diancie exists, we're just waiting for info on the other two.
