With 3 days you only have time for one location. As you can see the beach anywhere, I recommend just staying in Havana. With 3 days you can visit “Granma” and the Museum of the Revolution, check out some of Hemmingway’s old spots, catch a show at the Hotel Nacional, visit the forts at the entrance to the harbor, and wander the Malecon and the narrow streets of Havana Viejo.
Everyone will tell you to go now before there are McDonalds everywhere. I guess that’s good advice. I too wanted to visit Cuba while Castro was still alive and few Americas were visiting, so I traveled there before Bush's election and his payback to the Miami Cubans with the enhancement of the embargo. If anything, things are more dilapidated now.