If after replacing the spark plug, your still getting this code - Open up your camshaft and tighten every last bolt before it snaps. They come loose over time, its a manufacturing fault, eventually snapping your camshaft. Problem solved.
Vauxhall Corsa B - P0303 Cylinder 3 Misfire Detected B-22 - Avant
Moved to Technical. Thanks for the tip: this fault has been aired several times on here, but there may be people suffering from a Corsa (particularly 2005 models) who aren't aware of it. There may be some whose camshafts snapped without receiving this warning.