I don't think there's much point to using one half of the rails or the other, it would sound nearly identical either way.
What I would do is put three push pulls in place of all three pots (get 500k push pull pots). Have the furthest pot from the pickups split the bridge humbucker, have the second furthest push pull split both the neck and the middle humbuckers (you can split two humbuckers at once with one push pull), and then have the closest push pull make the bridge pickup "always on", so that it remains on in any selector position, then if you select the neck you get bridge + neck, and if you select middle+neck you get all three pickups at once.
With this scheme the only drawback is you'd have to split both the neck and middle at once, but IMO there's not a great value in splitting one humbucker and not the other, and you keep an all stock appearance since it requires only three push pulls. Some people might suggest drilling holes and putting in all sort of toggles, series, parallel options, but it gets out of hand and you end up with tons of sounds that are either gross, our sound barely different than the next.
What is your plan for the tone knobs? There's cool stuff you can do with that too if you're willing to go non-stock with the tone controls.
