> last year I have made a matpakke with 3 slices of bread
Which also aligns with what's said in the article. If you eat three of these in a meal, you're getting one more slice of bread than you'd get from a meal consisting of a single sandwich.
> Compared to other sandwiches the Norwegian sandwich seems to substitute protein (less meat and cheese) for more saturated fat and carbs (an extra slice of bread, butter on the bread).
It's meaningless to compare to a "single sandwich" anyway, because e.g. the bread size will vary dramatically depending on country. I live in the UK now, and the slices used for sandwiches here are often 50% or more larger than what I'm used to from Norway.
But especially if talking about the amount of protein relative to carbs or fats, I don't think it's very different - each of those three slices of bread gets their own slice of meat or cheese. And not by any means everyone will use butter, while you'll get plenty of sandwiches elsewhere (e.g. UK) that are buttered.
A Norwegian "sandwich" has one slice of bread. The picture at the top of the article shows 3 stacked open-face sandwiches.
Sometimes people will do "British-style" sandwiches when putting on things like jam to avoid making a mess, but open-face sandwiches is most traditional.