two-days trip from Oslo? (Flam train being closed down, need a change of plan)

if 14 (16 to the airport and back) hours on a train is your idea of a great 3 day trip then fine. Some of the trip is beautiful but I would not say it was all amazing plus this time of the year you will not see that much, you might get lucky with sunshine but it will be more than likely be wet cloudy and even in daylight hours gray and not appealing. Mid winter or summer would be the time to go for me although even then I would not on a such a short trip want to do a journey of that length, i am not sure why you did not just book a flight into Bergen if you wanted to see the Fjords.

If you still have some time to decide: You have the possibility of tsking the train from Oslo Gardermoen airport to Dombås, Jump on the Rauma rail to Åndalsnes (part by bus now, rail closed by fear of rockfall), Bus along fjords to Ålesund, plane (widerøe) back to Oslo. Find two places to rest inbetween, the route has a lot of hotels and other accomodation.