Simplest way is just a switch (maybe a push pot) to set the middle pup as the south coil on either the neck or the bridge, depending on what's selected. You're sending the hot from the middle pickup into the grounds of the other two pickups when the switch is on. Then the 'middle only' #3 selection on your standard strat switch becomes just silence, #1 & #2 become neck bucker, #3 and #4 become bridge bucker. Both of my strats have that option, one as a switch and one dials in from a tone pot. No guarantees on the quality of the resulting HB, but it is a strong, midrangey, and hum canceling tone on my strats.
I'm at work right now but i could draw you up something if you can't find a diagram in the usual spots.