Does Apple sell a protective screen cover/case for the watches (series 3) ?

Hi

Apple does not currently sell screen protectors or cases for Apple Watch Series 3 on the Apple Online Store.

All currently available products are detailed here, for the US, for example:

Buy Apple Watch Accessories - Apple

If you are in a different country, then select the corresponding page for your country.

To check which products are stocked by your local Apple Retail Store, you can contact them directly to ask:

Apple Store - Find a Store - Apple

(Note that you are not talking to Apple here - this is a user-based technical support community).

I don't see any but, you can search for yourself:

Buy Apple Watch Accessories - Apple