We are taking the north to south Havila cruise in August and would like to include inland exploration and viewing without a car. The first leg of our trip will include three days in Oslo, then a flight to Kirkenes. Would it make sense to take the train to Myrdal, to the famous Flam train, and return to Oslo in one day? Thank you.
In my opinion no. While it is possible, it's a bit over 6 hours one way, so a very long day trip.
Personally, with only 3 days in Oslo, assuming this includes your arrival day, I wouldn't bother with the train from to Myrdal, then Flåm UNLESS you include the ferry through the Naeyorfjord. But this would be an extremely long day.
Where are you disembarking on your Havila cruise? If Bergen, you could do the scenic trains, bus and ferry route to Oslo in 1 or 2 days.
You can also book the components yourself DIY Norway in a Nutshell.