Apple TV 3rd generation apps

Unfortunately, you can’t. The Disney+ app is available for newer models, Apple TV (4th generation or later), that have an on-device App Store. However, you can AirPlay Disney+ from an iOS device to Apple TV (3rd generation or later).

What devices and platforms are supported by Disney+? - Disney+ Help Center

Can I use AirPlay to watch Disney+? - Disney+ Help Center

Disney+ app - App Store preview - Apple

The 3rd gen doesn't run apps, only preset 'channels sent via server. The last provider to generate a server version was Amazon, which was a few years ago. Current providers have also been dropping support, with the most recent being Youtube and CBS.

The option would be to use the app on another Apple device and use airplay to forward the stream to your Apple TV, upgrade to a current model (Apple TV HD or 4K) or use another streaming device.