Take any standard wiring diagram that suits your pickups - make two separate ones - then instead of having two output jacks, run the 2 hots to a 3-way LP-style toggle switch (upper neck -> both necks -> bottom neck), and run the output of the switch to a single output jack.
It's no harder than that. If you're going Strat style, you don't even need two 5-way switches, you can just run off both sides of a StewMac super switch (which is what I did), and put the 3-way toggle in the hole where the 2nd tone control goes.