How do I operate 3rd gen Apple TV?

Do you have the Apple Remote (aluminum) that should be bundled with it?

If not, then you may buy one from Apple or other resellers.

Apple Remote (aluminum) - Apple TV accessories - Apple Store online

If you lost the remote for your Apple TV - Apple Support

Identify your Apple TV remote - Apple Support

Newer models remotes are not compatible with this model Apple TV box.

When all else fails, read the manual.

See Apple - Support - Manuals. Download the 3rd Generation Setup guide.