IMO, your cramming too much into what is basically three and a half days.
I guess if you really need to see Oslo, Bergen, and one short day in the Flam area this will be okay. I originally planned to take the one day tour in Flam, but after considering what was important to me, seeing the fjords, I decided to spend more
time in the fjord towns. So glad I did. Bergen was a beautiful city, but the fjord towns and boat rides are what I remember.
I'd reccommend spending at least two full days in a fjord town. When you say you'll start the nutshell tour on day two, you wont't get there until mid day, and you'll be there at the crowded time for boat rides. And if it's raining, this will be your only shot. If you spend a couple of days you can catch early and late boat rides and give yourself a better shot at good weather. It rains A LOT.
As for Flam, not a fan at all. Touristy and packed with cruise folks. I stayed in Aurland which was a short bus ride away. Very quiet and I stayed right on the fjord. Balestrand, as others have mentioned is much more pleasant than Flam.
I'd pick Bergen or Oslo and save the other for another trip. Too much running around.
I posted a trip report not long ago covering my visit to Bergen and fjords. Painfully long, you've been warned.