code popped up p0303 change the coil still having same problem engine shacking

http://www.autocodes.com/p0303_cadillac.html

P0303 is a cylinder 3 misfire. If this is the only code pulling and you have changed out the ignition coil, check to make sure you didn't get a faulty coil or plug first (this can happen with any part dealers parts, even brand new), if the coil and plug are both in good condition, have a trained professional perform a compression test to check your cylinders