Any suggestions for must sees in Stockholm for a three day stay?
Aside from the common recommendations that you can get from Google or asking Alexa, It depends on your interests
Most guide books will give a listing of the sights: Rick Steves, in his, gives “rankings” as according to what he considers the must-sees.
Be aware of pickpockets in Stockholm - different level than in Norway.
Summary of Rick's rankings at https://www.ricksteves.com/europe/sweden/stockholm but walking around the old town is also a high priority, not on the list.
I concur with MarkK. There are "beware of pickpockets" signs everywhere, even inside my hotel (Marriott).
In fact, one day I was eating lunch in a restaurant's patio and a central-Asian-looking thieve attempted to steal my backpack. He was very aggressive, but I tied my bags to the chair and the table as a precaution. Two days later, in the same neighborhood, I saw this guy with his sidekick inside a hipster coffee shop.
Vasa Museum is one of the highlights in Stockholm. Also, if you are an ABBA fan, you could visit he ABBA museum too. Another museum worth visiting is the Skansen open-air museum which is the oldest open-air museum in the world and also the Stockholm zoo, with animals native to Scandinavia.