You can get a one country pass for Norway yes.

More info in this page: https://www.interrail.eu/nl/interrail-passes/one-country-pass/norway

The Flåmsbana is not included in the pass, but it comes with a discount if you have the pass. It’s very touristy though. The Norwegian railway network is very centred towards Oslo, so for all your stretches you will have to return to Oslo to switch trains.

When are you travelling? Plan any other visits besides Bergen, Flåm, Oslo, Trondheim, Flåm. You might be cheaper with separate tickets

But if you only do Oslo-Bergen; Bergen-Oslo, Oslo-Trondheim and back to Oslo and book well in advance, ordinary tickets are cheaper than pass+reservation fee.

Ordinary tickets are available for about +/- 250 NOK one way. Which make you can do it for about a 1000 NOK pp (about €100 - excluding Flåmsbana, but that you can do with a “Norway in a nutshell” tour, combined with bus and spectacular sailing through Nærøyfjorden).

Check on EnTur.no.

EDIT: If you fly into Norway via Oslo Gardermoen, skip the (expensive) Flytoget trains and take an ordinary Vy train (R11 -L12).

Thank you so much really helpful

If using ordinary tickets buy the premium class option. You get free drinks and sometimes food as well. All the routes require advance reservation. The Flam railway is very scenic but you might have to share with cruise ship groups. I was in Norway for 3 weeks recently and had a brilliant time but Norway is very expensive so be prepared!