That's an old trick. When you hold the spark plug wire a little away from the plug you are in effect increasing the spark plug gap, which increases the voltage necessary for the spark to jump the gap. Higher voltage means - you guessed it - more intense spark and therefore your plug fires. I had something similar happen with an air-cooled engine - in that case it turned out to be bad points.
I'd first change the plug out - I've had bad ones right out of the box before. Autolite 303's work great for distributor systems, Autolite 295's for magneto systems in AC tractors. Next I would change the spark plug wire. Then I would replace the distributor cap and rotor.
Your #3 cylinder could have enough oil fouling that nothing fixes this, but if it's a simple miss and all other things are ok one of the above fixes should help. You don't want to increase the plug gap much - a higher spark voltage means higher voltage across the points - and can eat them up in a short time.
Hope that helps.