There is nothing illegal per se for a car dealership to sell a customer a car with three different sized tires in that the terms "legal" and "illegal" generally pertain to criminal matters. However, from a negligence point of view, if you were in an accident due to improper tires on the vehicle you acquired, the dealership could very well be liable for any damages that you might sustain as a result of the improper tires.From what you have written about the warranty issue, it seems that the car dealership may have pulled a fast one on you by not giving you the warranty as advertised. I recommend that you consult with an attorney who practices in the area of automotive law and/or make a complaint with your local department of motor vehicles who regulate car dealerships about the issue you have written about.