Under WA state contract law can a person back out of a new car purchase within 3 days

Washington contract law does not specifically address new car sales. But some dealerships have that within their agreement. If you have a particular reason you do not like the car because it does not have features you sought, you can request that the dealer rescind the sale. If there are not too many miles and it is still in new condition, the dealer may take it back, and can resell as a new vehicle (assuming the title has not yet been changed into your name). There is a body of law that protects consumers from sales practice violations, so that may be a way of requiring the dealer to take the car back.

You have not provided enough information to discuss what might be an appropriate remedy. You should contact a lawyer regarding your rights in this regard.