get netflix on 3rd generation apple tv

It's geographic based - where are you located?

on the ATV3 you don't have the ability to add/remove apps. However, it could be "hidden" in which case you can unhide (See link below). I would suggest making sure the software is up to date and then checking the hidden status...

Arrange and hide apps on your Apple TV - Apple Support

I've had Netflix for a few years on AppleTV 3 in the UK, but only since Netflix launched the service here.

Is Netflix active in your country?