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

Once you've got the car and the dealer/seller provides title (presumably received the money or the loan), the contract is "executed".

And the first step in any question like this, is read the agreement you signed and made.

Agreement may or may not be important. Go back to dealer or private party and ask them to take the car back. Even though the contract was air tight in dealers favor...Our local Ford dealer took back a Honda used car after one week, full cash refund...then asked us to browse around on a number of their lots to see if there is another car we want! I told this story to many people = much good publicity. On a new car, this is not possible because now it is a used car. If dealer refuses, select another vehicle from their lot that you will buy-- if you can return the first one. If still no go, offer a cash incentive- what is it worth to you to NOT have the car? If still no go, try to get the bank to cancel your financing because your application was fraudulently altered, you are unemployed, etc.