2004 Dodge Ram Code 303

Could be poor compression, faulty sparkplug/ wire and injector on no.3

Several things could be an issue. Since you already replaced the coil packs and plugs, I would recommend that you check engine compression on that cylinder to make sure it is within the manufacturer's specs. For only one cylinder to misfire, it is either an electrical issue, fuel injector issue or weak compression.

Tell me this. Can you feel the cylinder misfire? Does the truck shake?

I was thinking about your truck and had some after thoughts. Check these things too

---Check for Carbon Tracks on the Spark Plugs and inside the Spark Plug Boots. These conditions will cause the Spark to bypass the Spark Plug and jump to ground on the engine.

--- make sure that the Misfiring Cylinder is getting Spark.

--- The next check would be the fuel injector. Here are directions for checking them:
http://troubleshootmyvehicle.com/chrysler/3.9L-5.2L-5.9L/how-to-test-the-fuel-injectors-1
Let me know what you find. If you need directions for checking engine compression, let me know. Also, if you are getting a good blue snapping spark to the plug, you may have a partially plugged injector, sticking injector...