Depending on what sound you want to get out of you guitar, take the followinginto consideration.

Single coils pickups are naturally brighter than humbuckersand have their own distinct sound. The downside is that they will pick up humfrom alternating currents which will travel with the signal from the string toyour amp. This causes a much noisier effect when distortion is added. 250kpotentiometers (or "pots") are commonly used for single coil pickups.

Humbuckers are wired in such a way that hum fromalternating currents are cancelled out. An inherent feature of a humbuckers is that it has less treble and a fatter tone. Humbuckers also have a naturally higher output than singlecoil pickups. To compensate, 500k potentiometers are used for humbuckers because they will attenuate less high frequency.

Different magnet materials used in a pickup will give different tones. Alnico2, alnico 5, and ceramic are the 3 most common types.

Alnico 2 magnets are the clearest signal of the three. They also have thecleanest sound.

Ceramic magnets are the least clear and dirtiest sound of the three. They aredistorted and generate a rich palette of harmonic overtones.

Alnico 5 magnets are a good balance between alnico 2 and ceramic. They are bestfor players who switch between clean and distortion settings or just don't wanteither extreme.

String vibration is widest and most clear at the 12^th fret andmore distorted closer the bridge and nut. Because of this, commonconfigurations are for a guitar to have a ceramic magnet pickup in the bridgeposition and an alnico 5 magnet in the neck position or to have an alnico 5magnet in the bridge position and alnico 2 in the neck.

Neck pickups will have a lower output volume compared to the bridge. This isbecause the string movement is much greater at the neck. The pickup typedoesn't always change. It is more common to have two humbuckersof the same magnetic type in one guitar.

Active pickups run off of batteries. The sound is usually compressed andequalized by electronically shaping the sound of the pickup. They have moreoutput than passive pickups and will drive an amp's preamp stage into distortioneasier.

In a guitar with three pickups, the middle pickup uses the neck pickup as themiddle pickup as well, not the bridge. So it would be bridge/neck/neck. Mostcompanies don't make a middle specific pickup.