3D movies trick your brain into perceiving depth. Normally, your eyes are 2.5 inches apart, so each eye sees a slightly different angle. The brain fuses these into one 3D image.
An HD movie will "work" on a 720p screen, but you won't see the benefit. Conversely, a 720p movie on a 4K TV looks blurry.
So the next time you search "hd movies 3 work," remember: The hardware is ready, the files exist, and with this guide, you know exactly how to flip the switch.