The stock position 3 is the neck pickup and half the bridge pickup, which sounds like a cross between the middle pickup and the middle (bridge and neck) position on a telecaster. It's a very cool tone. Obviously you could re-wire the switch to get middle pickup if you want.
In the stock wiring scheme the silent circuit is only wired to the single coils, though it is only active in position 5, which is the neck pickup on its own. Another single coil can be added to the silent circuit, as is the case on the SSS guitars but I'm not sure if it can be added to one of the humbucker coils.
Be warned - the switch used is a 4-pole 5-way super switch (that's 24 terminals) and the wiring is complicated enough that modifications are really a job for those who are good with a soldering iron and are not scared by complicated wiring. One pole alone controls the silent circuit. If you spit the humbucker on a separate switch it will add an extra layer of complication to an already complicated circuit. And my guess is that you would need a triple-pole double-throw switch for it to work. And the silent circuit can be adjusted, it may require a different adjustment for half a humbucker compared to the singles, and may not work perfectly well with both.