You cannot add apps to the 3rd generation Apple TV. It comes with a variety of pre-installed apps and Apple adds new ones from time to time via software updates. You can, however, add apps to your iPad and stream them to your Apple TV using AirPlay.
Apple is expected to launch an App Store specifically for the Apple TV when the 4th generation Apple TV comes out, and a software development kit will also be released to third party developers. The Apple TV 4 is also expected to be the hub for HomeKit and all of its apps, so you will be able to install your own apps on the new Apple TV.