Just recently I have began to experience vibrations when I pick up speed. The vibrations are stronger in 1st gear and not as strong in 2nd and 3rd. The SES light flashed at me a few times then stopped. Last month I had both of my front wheelbearing assembly replaced.
Keep in mind that the computer is probably detecting faulty current reading from the coil pack to the wire attached to that plug. It "may" not be the plug. Someone correct me if I'm wrong but I don't see how the computer can detect a bad plug.
I read that the computer can detect an increase in engine RPM when each cylinder is on its power stroke. If the rpm doesn't increase when #3 cylinder should be firing then it reports the misfire. Hope that explanation makes sense.