Is the Series 3 Apple Watch still supported by Apple?

The Series 3 is still supported by Apple, though I suspect not for much longer:

Obtaining service for your Apple product after an expired warranty - Apple Support

But you don't get your cellular service through Apple. Are you trying to set up cellular for the first time? Or, did you already have cellular set up on the watch and it stopped working?

Apple no longer supports the Series 3 with software and security updates, you can still use it, but it will start to degrade functionally with age.

Learn how to ⇢ Set up and use cellular service on Apple Watch - Apple Support

-Zachyy ✌️

Can you give us more details about your devices, that you want to connect to cellular data?