Hi there! I did this exact trip this past August and it was a wonderful day. I took an early train and arrived in Helsingor before 10am, the ferry is under a half hour, I spent a couple hours in Helsingborg and had a late lunch in Malmo (I was lucky to be there for the festival so lots of yummy street foods). It's easily doable in a day and, in particular, I found the train ride from Helsingborg to Malmo to be very pretty. I was back in Copenhagen by dinner.

I had the CPH card so my train trip from copenhagen to helsingor was 'free' and i got a discount on the ferry, but there is an "Around the Sound" ticket you can buy that would include this whole trip. Not sure how much it costs, but it includes the ferry. I bought an inclusive ticket in Helsingborg that included the trains to Malmo and then back to Copenhagen, and I didn't write down the price in swedish krones, but it was about $23 USD, which I didn't think was too bad.

Anyway long story short, yes this trip can be done and I would highly recommend it. Gave me a great taste of Sweden and made me want to go back one day! Helsingor is a lovely little medieval town and it was fun to be there around 10am when everything was "waking up" - of course Kronborg Slot is beautiful - I didn't personally bother to go inside but I think you'd have time to if you want. Helsingborg is equally cute, there are a few pedestrian/shopping streets not far from the train station and even though I only spent about an hour there, I thought it was nice. I admit I didn't see much of Malmo since my whole time there I was wandering through the festival but it's definitely the biggest of the 3 towns and probably is worth more of your day.

Have fun :)

Greetings

It is doable, but how rushed you would be depends on the time of year and what you are interested in seeing in each location.

If you are there in the next few months, museums/castles etc. will be open shorter hours and you won't have as many hours of daylight. In the summer opening hours are extended and you'll have long, long evenings to enjoy.

It takes about an hour (or a bit longer?) to Helsingor and Kronborg probably won't be open until 10am or later. If you are into castles, it's probably worth leaving a couple of hours for the castle as there are several separate sections to explore (castle, naval museum, vaults etc.).

I don't know about Helsingborg or Malmo, but I think you'd probably be rushed if you wanted to see a bit of each city. Perhaps you could choose one Swedish city to see in more detail. You could do Helsingor in the morning, have lunch in Helsingborg and explore Malmo in the afternoon. Or stay in Helsingborg and have dinner in Malmo.

Have fun!

Kate