The bottom pin is 12v, the center pin is ground, and the top pin is a speed sense.(This is just a guess, I'm not really sure and I'm at work and can't test my p2 cooler at home)
It's not your standard 3 pin plug like you'll find on the motherboard, if you lost the cable that goes from the fan to the board you can.
1. Find another cable
2. Replace the fan
3. Solder wires to the pins on the fan.
You could tightly wrap the pins with wire, solder is preferred for a sure connection. Then attach it to a 3 pin connector to the board or attach it to one of the connecters that go to a drive.
Sometimes if the fan is too loud for me I shove the ground wire into the 5v part of a floppy connector and then connect the positive side of the fan to the yellow wire of floppy connector. This will run the fan at 7v which is fast enough to cool it but slow enough to be quiet.