score:5
Can I travel legally to Tromso with my Schengen Visa?
Yes.
Given that I have mostly stayed in Italy, will it be a problem if I go for just 3 nights?
No.
The flights between Italy and Tromso has a layover in Poland. While entering Poland, should this be a problem as well?
No.
What are the documents I need besides Passport, Visa, Tromso hotel booking and Tromso to Italy return ticket?
You might want something to read on the plane. Otherwise, nothing.