3rd day in Stockholm or 2nd day in Helsinski

Without knowing what you will see in Stockholm on the escorted tour, it's hard to say for sure, but not having a third day to be on your own in Stockholm feels like it would be a terrible mistake.

There's a lot more to see in Helsinki, though, than you could fit into a day. And Tallinn in just one day will feel rushed, in my opinion.

It would help to know what you mean by 3 full days. Assuming the tour ends with a last night in a Stockholm hotel, are you saying you have 3 more nights after that? Do you have a flight out of Helsinki already booked? It would be easier to help you with a tad more information.

Here is more detail on my itinerary, starting with the arrival to Stockholm:

June 29 - Arrival to Stockholm by night (I'm not counting this day)
June 30 - Stockholm - I don't have details on the included sightseeing, but there are optional tours of the Old Town and Vasa museum (both of which I will do for sure, whether with the tour or by myself)
July 1 - Stockholm - Tour ends with breakfast, so I have most of the day to spend in Stockholm by myself. Would take the night boat to Helsinki.
July 2 and 3 - Helsinki
July 4 - Tallin (arriving in the early morning and sleeping there)
July 5 - Return. I'm not counting this day. Unfortunately, I already have it booked to return from Stockholm so will have to take a short flight from Tallin to Stockholm and then comeback from there to the US.

The above has 2 full days in Helsinki. The alternative would be to spend July 2nd on Stockholm (so a third day in Stockholm and only one day left in Helsinki). Or could do Tallin on the 3rd and spend the 4th in Stockholm (resulting also in 3 full days in Stockholm and 1 in Helsinki).

I should also mention that I'm not into art museums, so I need less time in a city than other people would.

Thanks for the additional info. This is helpful to give you some guidance.

Here's what I'd suggest:

July 1: Stockholm, overnight ferry to Tallinn
July 2: Wake up in Tallinn, stay overnight
July 3: Ferry to Helsinki
July 4: Helsinki
July 5: Fly home (Helsinki -> Stockholm, Stockholm -> USA)

I suggest this order because you have two nights in Helsinki and one night in Tallinn, so do Tallinn first. Then you can decide how much more you want to see after the first day, and that will help you decide how early the next day to depart to Helsinki.

The way you suggest, your time in Tallinn will be cut very short. I think it's worth more than a day. You'll want a day to explore the old town, and then the next day (July 3rd) I'd recommend taking a bus out to the Song Festival Grounds, which I think is a must-see just from the perspective of the historical context and the meaning of that place to the Estonian people. Then you can walk from there to Kadriorg Palace and exploring the grounds of the garden and park, catch the bus or streetcar back, and hop the ferry. But if that doesn't appeal to you, you can always get an earlier ferry and head to Helsinki and spend most of July 3rd there.

I do think an additional day in Stockholm is worthwhile, but given the amount of time you have and your desire to see both Helsinki and Tallinn, I don't think you can swing it. Make the most of your days on June 30 and July 1, maybe ditching the tour so you can see things at your own pace. Rick Steves has two great walking tours in his Scandinavia guidebook. Do them both. And when you go to the Vasa Museum, hang out in Djurgården, maybe rent a bike. Stockholm is a big city and there's a lot to see outside the old town.

Have a great trip!

Thank you Lane! I like your suggestion. I'm really looking forward to see Tallinn, so doing it that way gives me flexibility to extend my time there if I feel like it. Thanks for the suggestions for Stockholm too!