Apple TV 3

The third generation Apple TV does not use Apps, it only has Server Pushed channels. There is no way to add or remove channels only hide or show them from the Settings app.

If you only see a small number channels. Check Settings->Main Menu and make sure the rest are not hidden.

As mentioned the channels are pushed from the server, so you see any and all channels that are being pushed unless they are hidden in the Settings menu.

Available Channels are region based, and as such not all channels may be available in all locations.

If nothing is hidden in Settings->Main Menu, then there's some other issue. Just pulled out my old 3rd gen Apple TV and I can still see many channels.

The only other option where channels may not show, is when the Apple TV does not have an internet connection. However, in that case generally only the Settings and Computers channels are visible. And you get a message to that effect at the top.

Which apps do you see? That may give an important clue.