Hey,
I just recently bought a 1997 Camaro V6. it had the check engine light on so i ran it to an autozone to get the codes pulled. It pulled code p0303 or a cylinder 3 misfire. I replaced the spark plug, wire, and put some lucas fuel injector cleaner in my gas tank. after about 3 days of driving the check engine light went off and all seemed good to pass the emissions test and get her plated. this morning i ran her over to the emission testing place and as I waited in line( idling in drive so the engine stayed warm) the light came back on. Needless to say i failed the test and decided to run it back to autozone where it pulled the same code. The guy told me its most likely the coil pack or fuel injector. anyone that can offer any advice is very much appreciated!

 

Try swapping the coil packs & see if trouble moves to a different cylinder. Also check the plug on the misfiring cylinder, and compare color with the other plugs.