Watch 3 question

No.

There are 2 Apple Watch cellular radios

• Canada, U.S. Mexico, Puerto Rico, Dominican Republic, Ecuador

• The rest of the world

Carriers will not attempt to connect to an Apple Watch that does not use the correct radio.

If you do not get your Apple Watch in your own country, you may have to return the Apple Watch to the country where purchased to get warranty service.

If you do not get your Apple Watch in your own country, there may be country specific restrictions in the installed Apple Watch software that prevent features from working when you get home.

https://www.apple.com/watchos/feature-availability

Maybe. The iPhone and watch MJST use the same cellular provider and you must be within your carrier's network when you set up cellular on your watch. Here are a list of cellular network providers that support the Apple cellular watch in Europe:

Apple Watch - Carriers - Apple

A cellular watch bought in the US will only function in North America,Canada,Mexico and Puerto Rico anywhere else it will function as a GPS only watch.