Is there a law that gives you 3 days to return a car you just bought in Florida?

No, those laws are called, cooling off period laws, or buyers remorse laws. Those type laws do not apply to the purchase of a vehicle of any kind. It is only a myth that they do. Once you buy a car it is yours and you cannot return it. There are lemon laws in most states that protect the consumer against manufacture defects that cannot be repaired. In that case you can get a replacement vehicle if you follow the rules.