HELP: Misfire Cylinder 3 (3.4L V6 PreRunner)

Ok boy & girls, I need your help to figure out WHY I can't get rid of this issue...

The Truck:
2003 PreRunner XtraCab (3.4 V6)
91,000 Miles
K&N Cold Air Intake
MagnaFlow Cat-Back

The Problem:
Misfire: Cylinder 3
Started around 80,000 miles

What I've Tried So Far:
-Cleaned/serviced Air Filter
-Cleaned MAF Sensor
-Replaced all plugs (with OEM Denso's from Dealership)
-Swapped Coil Packs (in hopes the misfire would jump to another cylinder)
-Replaced all injectors (with OEM units from Dealership)
-Added SeaFoam to Gas tank (on multiple occasions)
-Checked plugs after new injectors were installed (clean)

Each step I tried yielded the same results;
Engine ran smooth for a bit (never more than a day) followed by the CEL: P0303

I've always been on top of suggested maintenance, either personally or handled by the trusted mechanics who work next door. I never run anything less than 89 Octane (Mostly Chevron, with the occasional Shell visit).

PLEASE HELP!