3rd gen apple tv

Not directly.

The Disney+ App is only available for Apple TV HD (4th Gen) and Apple TV 4k (5th Gen).

You can however Airplay Disney+ from an iPhone, iPad or iPod Touch to the Apple TV 3rd gen.

You may have to check the sleep / screen settings on whichever device you use to airplay from in order to prevent it timing out in the middle of a show. (Took us a while to work out how to stop this happening.)