Savannah and Charleston are very walkable cities. Atlanta not so much. But MARTA gets you several places of interest.
What are your interests? And are you talking hotels or AirBnB or combo?
Charleston anywhere in historic city center area. Same for Savannah. Though we stayed across the river at the Westin and just took the ferry across.
Downtown Atlanta has a lot of interesting things to do during the day, but not so much at night. You will probably end up driving more there.
Also both Charleston and Savannah have interesting oceanfront parks close by.