Reminder: I'm not responsible for any damage you may do to your phone. Proceed at your own risk!
Root is Required!
1. To root your At&t Note 3 follow this guide (Thank You designgears!)
2. You will need to allow unknown sources to be installed.
Settings>General>Security>Unknown Sources (Make sure its checked)
3. Install Chainfire's RegionLock Away App from Google Play Store (and buy him a beer ) or download it from here.
4. Tap "Region Unlock" in the app
5. Let it do its thing
6. Choose Reboot Later (Thats what I did at least)
7. Turn off the phone and try a non-At&t sim card.
I tried my old T-mobile sim in mine and it pulled up, and we just got back from the MetroPCS store and have his running on that.