Apple Watch 3 unable to receive calls on cellular but can make calls

Check you have an active cellular connection when the iPhone is shut down. The easiest ways, in the Explorer face you should have one or more green dots in the screen, and if you swipe up for Control Center the cellular Data icon will have shifted to a green background to show it is active. (If it's black, tap it and enable cellular data).

If you have a green cellular connection to your watch and you can't receive calls, check in Control Center to make sure you didn't activate any of Silent / Theater / Do Not Disturb modes. Then call your network provider and get their help because the calls are probably not switched correctly and not reaching your watch - that's most likely a problem their side.

Is the software up to date ? Also did you try turning airplane mode on and back? Make sure do not disturb is turned off. Go in your settings in your iPhone And make sure calls on other devices is allowed when your phone is not nearby