Norway in a Nutshell - Luggage

You don't say if you are doing the NIN roundtrip from Oslo.
If you are, then you could leave your luggage with the hotel you were staying at and collect on your return.

Distances between modes are all a short walk. The Myrdal connection is the shortest with a walk across the train platform from the Bergen Line to the Flam. Longest walk will be from your hotel to the Oslo S and/or Bergen train station to hotel.

If you are traveling with carry on luggage, your bags should fit the train overhead racks.

I'm sorry - I thought I had included that. I meant to say 'one way' from Oslo to Bergen.

And we'd have normal luggage - since its going to be a three week trip.

Normal luggage to us is a 21 inch rolling bag and a small backpack. We can travel indefinitely with it.

But other's normal luggage can be a 50 pound huge suitcase.
Our family rules are everyone old and young handle their own luggage.

https://www.fjordtours.com/customer-support/frequently-asked-questions/

You can bring your luggage with you on the trip. There are places to store the luggage on each transport leg.
Be aware that you have to carry the luggage yourself from train to bus to boat etc, but the connections are all within close proximity for easy transfer.
Change train for the Flåm Railway or for the Bergen Railway. The trains are located next to each other.
The fjordcruise boat terminal is situated next to the railway station. The fjordcruise boat for Norway in a Nutshell departs from pier no.1
The bus for the Norway in a Nutshell park next to the harbour and connect with the tourist boat/fjord boat.
The bus to/from Gudvangen arrives/departs at Voss Railway Station. The bus is located outside the railway station.

Thank you, Edgar! We really appreciate the information!!!