Assuming you want to use two pickups and have both on in the "middle" position (with a single volume pot), you can use the diagram I attached. (I only marked the top part of the drawing since it would be the same for the rest of course.)
Here's how it would function:
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Position 1: bridge
Position 2: bridge and neck
Position 3: neck
The ground wires of the pickups will go to the pot's ground, not on this switch. This switch will only be used for switching the "hot" signal from the pickups to the input of the volume pot.
Also note this is an unusual switch. If you buy a 3-way DPDT, it's possible it won't function the same way as this (the middle position might be "off"). Be sure you're using the right sort of switch if you expect this to work for the middle position.
I hope that helps!