How do I add apps like CNN to a 3rd Gen AppleTV

You can’t add anything that isn’t standard available.

To add apps of your choice, you will need an Apple TV (4th generation or later).

CNN stopped supporting the 3rd generation model in early January 2021.

+1, in short you need to buy a more up-to-date Apple TV. I recommend getting an Apple TV 4K and replace the 3rd generation.