position bass + middle --> gives a very bluesy warm sound, try rolling off the tone of the middle pickup also, it makes it darker.

middle + treble --> good for backing rythm, or funky solo. Used with overdrive, it gives a thicker sound than the treble pickup alone.

middle alone: not very used, i know that mark knopfler uses it a lot. It's nice, but doesn't have much personality.

I feel stupid and genius at the same time explaining all this, because you have the guitar in your hands, so you could try it all by yourself ...