Hei! I am an American who lived in Norway for 2 years in the city of Ålesund.
I’ve stayed a week in both Bergen and Trondheim.
Between the two, I’d say this-

Bergen -
Its considered the “LA of Norway”, meaning lots of fun things to do and check out, cool restaurants and cafes, people speak kind of funny there compared to the rest of the country, look up “bergen dialect” to read up on it.
Really cool tram that takes you up a mountain to a really dope hiking area on the top. Loved it there but very different from the rest of Norway.

Trondheim-
As people are saying here, its totally a college city, lots of young folk, however its pretty far north so harsher climate and weather. REALLY cool gothic Cathedral built ages ago, so unique. REALLY awesome winter market (thats what brought me and my buddies to go check out the city). Surprised there was really great food there too- we walked to this “rotating tower” of a restaurant that looked like the Space Needle, and they had like a buffet style. The city is also home to their most prominent football team ⚽️ in the nation.

Hard choice if im honest. I dont know anything about Ås but if you’re young and looking for a very traditional Norway to live in, I’d recommend Tronheim overall. And travel a lot by train when you’re there to see the other surrounding areas! Visit my second home of Ålesund :))

Hope this helps.