No cure for misfire in cylinder 3

I have a 4 cylinder, 2003 Mitsubishi galant with 64000 miles. Several months ago the "Service Engine Light" came on. I took it to Auto zone and they found a misfire in cyliner 3. First we replaced all spark plugs and reset. SEL cam back on went back to Autozone, same thing. Changed wires, Cylinder 3 coil, and O2 sensor. SEL back on. Went to Mitsu dealer, they said it needed throttle body cleaning, and other fuels system cleaning. Used Seafoam and worked good for a while. For a month the SEL would go on for a week and off for about a week. This lasted for about a month. Went to dealership they said needed reflashing. Sel light came back on after a few hours.