Unfortunately I think you may be mostly disappointed. The Baltics have gone to great lengths to try and erase visible traces of their Soviet history (especially monuments and such) and you will likely be very hard pressed to find any museum or historical exhibition that is not filled with anti-communism, anti-Soviet atrocity propaganda, historical revisionism and even glorification of fascists and Nazi collaborators.
I hope that i am wrong and you do find something interesting and not completely hostile to the Soviet period, but i would at least advise to manage your expectations.
That's not to say that the Baltics don't have interesting things to see but if you are interested in socialist history you would have a much better time visiting Belarus.