Under FL contract law can a person return a used car within 3 day windown

No. You cannot return it unless the dealer has a "cooling off" policy that allows you to return the vehicle and get your money returned without questions asked. Also, used cars are typically sold AS IS. Check the buyer's guide posted with the vehicle when sold. If sold AS IS you assume responsibility for any defects. Note that there is federal law that allows contract rescission after 72 hrs but this applies to in home solicitatons and certain types of loans. The federal law doesn't apply to vehicles purchased from a dealer.

The information I have provided is general information only and not intended to be legal advice. Please check with an attorney in licensed in your state for specific legal advice.

However, if it is a reputable dealership and you have a good basis for return, they might be willing to work with you, for example, offering another vehicle in exchange. It doesn't hurt to ask but the sooner the better.