misfiring 3 ove6cilenders

Don't know what year but this should cover it. P0300 FORD - Random Misfire Detected...Possible causes; Faulty spark plug (s), Faulty ignition coil (s), Clogged or faulty fuel injector (s), Intake air leak, Fuel injectors harness is open or shorted, Fuel Injectors circuit poor electrical connection, Ignition coils harness is open or shorted, Ignition coils circuit poor electrical connection, Insufficient cylinders compression, Incorrect fuel pressure...The P0300 code means that a cylinder(s) is misfiring or is randomly misfiring. Start by checking for intake leaks, intake gaskets are common caused of multi-cylinder misfiring. If no leak are found the next step is to replace the spark plugs. If the problem persist more tests needs to be done to diagnose problem.

Year, engine, miles? When were the sparkplugs last changed? If this engine has coil-on plugs, when were they last replaced? Is the check engine lit? What are the trouble codes?

Six cylinder, ok. Is # 3 cylinder misfiring or 3 cylinders. I've owned 3- 4.0's. Check the net to see how to check the coil (if you have just one) or how to check all six if you have a coil on each plug. If you have one coil with six wire connections check the resistance on each plug wire and see if they are within ford specs. For some reason, if they are out of spec, even slightly the plug will not fire.