I was given iPad3 as a gift. How do I find out what operating system it’s using?

No, that is a Mac computer operating system. You can open the Settings app on your iPad, tap General, tap About and then look at Software Version. The latest iOS that the iPad 3 can run is 9.3.5 (or 9.3.6 if it is a wi-fi + cellular model).

No. macOS runs on Macs.

iPads run iOS (now iPadOS).

On the iPad, go to Settings>General>About.

You will see the Software Version there.