How can I download Disney on my Apple tv

The Disney Plus app can be accessed and downloaded using the app store on the Apple TV HD (4th gen) and 4K (5th gen). Older models do not run apps, only preset 'channels' sent via server.

No problem. The 3rd gen would be over 8 years old and has vastly different hardware so it wouldn’t be possible to run apps or the TVOS system that runs on the newer models.