The NIN site does not cover breaking the trip Aurland. This means a DIY trip.
Must be booked:
Regional train Myrdal – Oslo and if you choose so, regional train Bergen – Voss (mandatory seat reservation)
Can be booked and can sell out:
Boat Gudvangen – Aurland – Flåm and train Flåm – Myrdal.
No booking available:
Commuter train Bergen – Voss (ticket vending machine at the platform)
Bus Voss – Gudvangen (pay the driver)
“Fjordcruise Nærøyfjord” covers one way between two ports.
Between Aurland and Flåm it's also possible to travel by “FlåmExpress” or shuttle bus.
One way tickets on the Flåm railway in only available through nsb site up to 90 days before travel date.
Both 2nd and 3rd are heavy traffic days in Flåm area but the cruise ships leave 5:30 pm on the 3rd. I guess Flåm 4 pm, Myrdal 6 pm is your best choice.