knocking sound in the engine

Knocking sound is not good (shows signs of mechanical engine damage). You are told oil is getting into the cylinder and fouling the spark plug and the code stored is most likely misfire number 3 cylinder. A compression test and or cylinder leakage test may be needed but it sounds like some engine dis-assembly will be needed to determine the extent of the engine damage and repairs needed.

Check for a loose spark plug on #3 first. They make a knocking noise & a miss when loose.