Cylinder three misfire detected

My car has the Duratec, it runs excellent, no smoke no oil burning no loss of coolant or overheating, it has plenty of power, idles very smooth, no signs of any troubles.
the check engine light came on but not flashing and the code is cylinder three misfire detected.
i replaced all six spark plugs and boots and the number three coil, I erased the code and after a few miles it came back.
all six plugs had wear from mileage, but no fouling or discoloration from being rich or lean.
i do not know why it is saying there is a misfire when the engine runs great.