The guys are giving some good info/advice for sure.
Since the issue is only on the #3 cylinder: I'd start by swapping the wires are the coils between cylinders 3 and 6 to see if the issue moves to 6. It's not super common, I have though seen an ICM (ignition control module, part under coils) fire only one cylinder of the coil in the past. Each of our coils fires two cylinders, which is why you can swap 3 and 6 w/o worries.
Head gaskets are pretty rare to go bad on this motor. However it is a gasket.
