Apple watch

All GPS only watches will work with the 3 network,the cellular version of the watch will not work with the 3 network,here are the carriers that support that support the cellular watch;

Carriers that support the apple watch

The difference is: one has cellular / mobile network and the other one does not.

What that means is, it can make and receive calls and send and receive messages and have internet access while away from the paired iPhone or a Wifi network.

Otherwise, the GPS-only model, requires being nearby the iPhone, or a WiFi connection to do those things.

See here: Use your Apple Watch without your iPhone nearby - Apple Support