Clam Fandango

I'm not sure of anything at this point. However, the usual signs of head gasket failure aren't there, like overheating or milky looking oil. Aside from looking for a coolant leak(there are no leaks), I'm not sure what to look for or how to check an intake gasket.

First let me start by saying I have searched and didn't find the answer and I'm new here so I hope I've posted I'm the correct place. I have a 2007 base 1500 P/U with the 4.3L V6 that just started having rough acceleration and rough idling issues. Its throwing codes P0303 (misfire in #3 cylinder) and P0036 (O2 sensor). I ran a tank with Seafoam through it and I have changed the wires & plugs and that didn't fix it. I have no experience with a distributor or the spider injectors that this truck has and before I go and attempt to replace the injector or take it to a mechanic I cant afford, I figured I'd ask here if anyone has any experience with this problem. Could the issue be injector? Or the distributor? Or am I going the wrong direction and the O2 sensor issue causing the roughness in idle and acceleration? Any help and/or advice is greatly appreciated, and thanks in advance!