Diagnosing misfire P0303

Hi, I've been working to diagnose the problem thats causing my truck to misfire. Any idea what might be causing the issue? If so, what would a mechanic typically charge? I've gone through most of the repairs I'm comfortable doing so depending on the issue might be looking for a professional.

Vehicle: *2014 Chevy Silverado *V6 4.3 *120k miles

Issue: *Code P0303 *Cylinder 3 misfire *White smoke from exhaust

Steps taken to repair: *Spark plugs and wires replaced *Coil packs switched, not the issue *Fuel injectors replaced