In a disappointing bit of news, it looks as though Disney has no plans to bring further updates to Disney Infinity 3.0 for the Apple TV. A Disney Infinity moderator confirmed the news on the game's online forums (via TouchArcade):
The team is currently focusing on the traditional gaming platforms. We are always evaluating and making changes, but there are currently no plans for further updates to the Apple TV version of the game. Please keep an eye out for further news about this, and feel free to contact Support if you have any questions.
Essentially, this means that those who purchased the $100 Disney Infinity 3.0 starter bundle for Apple TV won't be able to use any new play sets or characters in the game. That includes the new Marvel Battleground figures and any other forthcoming play sets.
It's entirely possible that Disney could change its mind and release updates for the platform in the future. However, for now, it looks like Apple TV players are out of luck.