I have a 3yr york hvac unit on really hot days it quits cooling and won't kick back in

My first thought is that you unit is over heating on the really hot days, my suggestion for stopping this problem is to start by cleaning the outside coil. Turn off the power to the outside unit, use a water hose and wash the outdoor coil, making sure you get rid of dirt, grass clippings, pet hair and if you have a dryer vent near by get rid of the lint and move the dryer vent with a piece of 4 inch pipe to get it away from the unit.
Next suggestion is on the days that you expect it to be really hot use a sprinkler to sprinkle around your unit for about five feet or so, this will give some humidity to the air and will cool the unit some as the air goes through the coil.

Let me know if this helps and if it does please rate my answer.