2001 F-150 0303 misfire detected

I highly doubt that all 3 coils changed could be bad which leads me to believe that the wiring going to the # 3 coil is intermittently opening or shorting out. Trace the circuit from the coil all the way to the cars computer/(control module. I not sure if your car is equipped with this) Check all connection for corrosion, dirt and opens. The voltage/current for this circuit is low untill it goes through the coil pack so any little drop in voltage can cause the misfire.
Hope this helps Backyardmechanic :)