I'll be in Stockholm and I definitely want to take the ferry across to St.Petersburg, but I don't know if I'll have enough time to fit Moscow in and make it worth it. Plus I kind of have Tallinn in the back of my mind for this trip as well. So who knows.
If you are in Stockholm, just do St. Petersburg and Tallinn. Both are worth visiting.
And if you have an extra day, you can always add Helsinki. Leave Moscow for another trip.