Is the Apple Watch SE compatable with the network Three?

The Cellular + GPS Apple Watch shares the iPhone's phone number, and the carriers that support the Apple Watch charge a monthly fee. In the U.S. the fee is $10 per month. Here is a list of carriers that support the Apple Watch:

https://www.apple.com/watch/cellular

If it's the GPS only watch then yes,if it is the cellular version then no.