If you own a MasterCard or Visa card you will hardly need cash when travelling in Scandinavia. Even small purchases can be done by card. Transportation tickets for bus or train can be bought by card or app.
Most guests do not need to go through the hassle of exchanging to different coins or notes. (DKK, SEK or NOK)
If you still like to have cash you can use EUR or USD notes in all counties.
EUR coins can be used in Finland and some shops.
Tour guides ought to have local coins for toilet stops.