Just to clarify, have you reset the engine lamp and it returns with the same code?
Can you feel the missfire, or is it just the code setting?
First we need to verify the injector is getting the pulse signal from the PCM (Powertrain Control Module). You need to place a noid lamp accross the injector harness with the connector removed and run the engine while watching for the lamp to pulse. If it doesn't we may have a bad injector driver internal to the PCM.
If the problem wasn't apparent prior to the replacement of the spark plugs and is now, than we could have a diffective plug.
You may also have a spark issue on this cylinder. Pull the coil wire at the coil on the #3 cylinder. Gound the end of a test lamp and place the probe end approx. 1/2 inch from the coil tower and inspect for good spark while cranking/running. You can also swap the 3&4 coil with the 1&2 coil, reset the Check Enginre Lamp, and watch to see if the missfire than sets on the 1 or 2 Cylinder, this would verify a bad coil pack.
If all the checks above pass, you may have internal engine problem and first a compression reading has to be obtained on all the cylinders.