fan wires

[Pepicek]

pulled several 120 mm fans from an old netserver (bought this case just for the fans)

It has a three wires: red, black, and blue

red and black for power (they are 12VDC). what is the blue for?

 

[chuckshissle]

It could be for tachometer/rpm sensor. Most stock pc have 3 wires or 4 wires in it's fans. Usually, red and black wire is power, yellow is tachometer and another black or blue is ground. My stock pc has this 4 wire fans.