Oslo -> Stockholm -> Helsinki in 9 days / realistic?

Hello,

Is it realistic to stay in each capital for 3 nights and then fly to the next capital. do I have enough time to explore the towns without too much stress?

2-2½ days / town : OK to see SOME highlights - but unstressful? NO "enough time": Not in MY opinion.
- especially too shoirt for Stockholm.
And you completely miss the opportunity to go oon daytours to the most important things like Skärgården (the archipelago).

Why are you so facinated with capitals? Maybe arrive Bergen + a quick Norway in a Nutshell to Oslo and flying on directly would fullfill YOUR interest much better??

yes although I would not fly from Stockholm to helsinki but take the ferry that way one of your nights can be on the boat and you get to cruise through the archipelago. You will not see everything in each city but 3 days is enough time to get a feel for each city, Personally i would opt for more time in Stockholm and helsinki and lose a day in Olso but that is only my personal suggestion

You can get a taste of each city this way. #2 is right about the ferry from Stockholm to Helsinki but I disagree about Oslo - Bygdøy museum island in itself is worth a good day of your time.