To properly visit and enjoy Elba, a car is usually reccomended. People with alot of time to spare can get around it by walking. For 3 days i would say walking is possible but not advised.
You can take your car there with the boats. Prices vary depending on date, time of day and car specs (bigger cars=more expensive). Fairs range from 50 to 150 (these are extreme's) and average around 80 euro for a medium car + extra for passengers.
Last, 3 days will be enough, but it might not be easy leaving not matter how long you stay xD!