p303 is a missfire in #3 cylinder-since you adresses the ignition wires and spark plugs -verify spark output from the coil just to make sure.Since you say it runs fine(the compter is not seeing the power of #3 when its suppose to fire

Fuel mixture fault (vacuum leak or wiring problem to that injector)-ingition or a mechanical fault have to exist

I am not there and I am not the one that worked on it-so if I was,I would have to diagnose it and would do so in this order since you dont have a scanner-check #3 spark plug (gap and prcelein for cracks)and possible move it to a different cylinder to see if the miss follows-with the plug out do a quick compression test on the 2 cylinders as a comparision of reading-look at how the plug is burning as compared to the other as well as the gap-(make sure the plug wire is fully seated and snaped onto the plug and there is no corrosion at the coil towers)then check the coil out put at the coil tower to make sure that 1 side is not weaker then the other.No whats left is fuel.You need a scanner to see just what the fuel trim is reading to see if its lean-since the injector has been replaced and this was ongoing before the repair?That leave a wiring or vacuum leak issue and for the wiring or command signal you need a lab scope to see the signal wave pattern and vacuum leak the fuel trim will show up as lean. Make sure the wires are not crossed so your not chasing your tail-the coils are marked-1-4,6-3,2-5 odds are in the rear