2002 chev express 3500 with the 5.7 192,000km's.

I seem to be getting a p0300 with a random P0303 which I understand is a random misfire and cylinder 3 misfire.

I have replaced plugs (AC DELCO), wires, cap and rotor and still getting these codes.

I seem to be suffering a bit on fuel mileage as I am getting 450km to a 110litre tank and although i do not feel any symptoms, poor fuel mileage is saying otherwise.

The ses light only comes on at highway speeds.

I have had the truck for 7 months now and the day I got it the check engine light came on to which I thought no problem, simple tune-up.

Now 7 months in still getting the dumby light.

I did notice when changing the cap and rotor someone prior to me did the intake gasket and the distributor was loose and did appear to be installed on a slight angle and was loosing oil out of the housing.

Timing was adjusted as it was way out and the distributor was repositioned and tightened down to which I thought would correct the problem.

I am now under the assumption the distributor drive gear could be the culprit and have done a tad of research but do not understand how a worn or damaged gear could point towards a p0303. I could potentially see the p0300 but not directly associated with the third.

I am mechanically inclined but do not really feel comfortable setting the truck to TDC and pulling out the distributor to inspect the gear.

I guess my question is, with what I have addressed above and only getting the referenced codes with no symptoms accept poor mileage, would this indicate a worn/damaged drive gear? I suppose doing plugs six months ago, these may now be fouled again and would suspect having to replace again.