Right, this is probably gonna act more as a bump so the bigger guys can answer properly but here goes.
As far as I know, there are two different types of 3 conductor wire pickups;
Tappable single coils, where the third conductor separates the pickup into a shorter wound coil, giving a different output and tone
Humbuckers, where the North-Coil end wire and South-Coil start (usually wired together for series humbucking) wires are combined into one wire and sent out of the pickup, tapping the pickup to one coil when connected to ground.
I don't know how you would go about finding out which of these it is, or which colour is which conductor. Is there a brand on the pickup? Here is how you would wire it up using a SPDT switch to utilise both sounds of the pickup, for those two scenarios. If you wanted just one sound from the pickup, use the main coil wire or hot wire only, going to the switch or pot, tape of over the extra wire and leave it.
http://img.photobucket.com/albums/v86/lala...lalala/bass.jpg