What is wrong when its telling me cylinder 3 is misfiring?

PO303 misfire code on the 4.3L engine 'most commonly' indicates a failing spark plug wire, however more possibilities certainly exist! .. Suggest further hands-on testing to prove cause of misfire .. Recommend replacement of > distributor cap - rotor button - spark plug wires and spark plugs, using *ACDelco* parts... Due to poor accessibility and difficulty of parts replacement, suggest seeking qualified hands-on help... Don't know your skill level or tool box arsenal.