(UPDATED 02/26/16 – Corrected the version number of the update, added a link to Apple’s webpage.)
The good news is Apple hasn’t completely abandoned the third-generation Apple TV, and is still releasing updates for it. The bad news is, we don’t know what’s in the update, because Apple isn’t telling.
8.4.1 7.2.1 (12H523) is available for all versions of the third-generation Apple TV. The update can be downloaded over the air, via the Software Update option in the Settings app. It will be delivered automatically for units that customers have enabled automatic updates for.
Apple continues to sell its third-generation Apple TV, which debuted in 2012, right alongside the new app-enabled fourth-generation box. While the older box doesn’t offer tvOS apps, or offer Siri support, it does offer content via such services as iTunes, Hulu, Netflix, and more.
View the security fixes offered by this update, here.