Placement changes the frequency response dramatically. Closer to the fretboard means deeper resonance, closer to the bridge means higher. What this means for guitar players is that you will have a more direct punchier sound near the bridge and a more mellow sound near the neck. How this applies to what players do with them depends entirely on the genre.
More pickups means more options, but most of my guitars only have 2 pickups and I never use the middle one in the guitar that has 3.