Apple Watch Series 3 (GPS + Cellular) comes in three regional models – Americas, China, and Europe/Asia Pacific. Unlike iPhone, there isn't a worldwide Apple Watch Series 3 model that supports all cellular bands used globally, so it isn't possible for Apple Watch to roam.
Since you purchase the Europe/Asia Pacific regional mode it will work with your Norwegian carrier Telenor, when or if they provide support for the LTE Watch in 2018 or later.
Dave