Question

2003 Ford Mustang 3.8L code P0303 cylinder 3 misfires changed plugs/plugs wires also

Answer 1

Are there any driveability problems? if the engine running rough, and you have already replaced the plugs and wires, then you may have a problem with the injector. Before replacing any parts, try cleaning the injectors.  You may clean the injectors by performing an injector flush (any shop can do that for you) or adding an injector cleaning solution to the fuel tank. if the problem persist you may have to replaced the injector.