Depends on how you made the connection with those plugs, but by the way you describe it, your mill will be disconnected for a moment before you short it. Just beware that once it is open-circuit the volts are much higher than battery voltage. It also starts speeding up as soon as you disconnect it and the volts at the prongs go up even higher. That's a high-voltage situation and you MUST plug in the short very quickly. Don't bump the exposed prongs on your finger, either. The jolt will make you take even longer to get them plugged together. All the while the WT is still speeding up while you stand directly below...
If you have a DPDT switch, you can wire two of the phases on the two poles at the center position of the switch, and the third phase wire to a pole that is NO and jumper to the other pole that is NO. When you throw the switch, the two phases on the center position both contact the third phase on the NO poles, shorting all of them. There are other ways to do this, such as with a 3-pole switch, but you get the idea.