The evidence is two leaks, the first of which is an image found in data files for the game. A piece of artwork - presumably the loading screen art - clearly shows Gen 3 monsters Duskull, Dusknoir, Shuppet, Sableye and Banette.
The second leak is Apple's doing; an image showing Gen 3 Pokémon - as well as a witch hat-wearing Pikachu - has been spotted on the iOS App Store. The Pikachu will presumably be a time-limited offering, as has been the case previously.
While we've yet to hear any official confirmation that Gen 3 is coming this Halloween, these two leaks would suggest it's almost certainly on the cards.