just curious, when you had your car washed did you have the undercarriage spray/wash option picked?

only asking because the rear of the motor is not really protected from the elements like the front is.

If the jet spray hit the rear plug wires and rear o2 sensor wires just right, I can see it possibly getting water in places you don't want.

But then now that I think of it...you said #3 was misfiring..thats the front middle cylinder.

get yourself a spray bottle filled with water and with your car running spray water around the coils, wires and plugs to see if it recreates the misfire.
If it does than you can go from there...replace wires...... plugs.....get my drift?.?.