CEL problems

I was getting catalytic converter efficiency, high egt and a misfire on cylinder three CEL's so i have the things to take care of the egt code (resistor and bolt), i have the cel override for the 02 sensor but the misfire code...little stuck on this one...?

the car runs fine and idles, no missing type action when i rev it up and all wires are grounded and everything

Mods:
HKS down pipe
Helix up pipe
K&N drop in
Grounding mod (took this off and problem still came back)
3/16 vaccum mod

i run 93 octane and Quaker State Q oil, 5W30, full synthetic

thanks in advance

Do you know the code?

P0303