I thought two would be enough + and -

Could anyone explain what the 3rd one does? Thanks!

https://kin-phinf.pstatic.net/202206...PEG/BP17_1.jpg

It's an I/O line for communication.

Signal output to measure RPMs.

As per https://superuser.com/questions/1126...the-difference below is the breakdown of the wires (3 pin & 4 pin):

A three pin connector is basically power (5/12 volt), ground, and signal. The signal wire measures how fast the fan is moving without any controls for the fan speed. With this type, fan speed is typically controlled by increasing or decreasing the voltage over the power wire.

A four pin connector is a little different than the three pin connector as it has the extra [fourth] wire for control to send a signal to the fan which likely has a chip on it that tells it to slow down or speed up in addition to the other wires just as the three pin connector has.

Three Wire and Four Wire Fan Connectors

Chassis and processor fans use either a three-wire or four-wire connector. The three-wire connectors are for small chassis fans with lower power consumption. The four-wire connectors are for processor fans with higher power consumption.

Three-wire fan connecting to a four-pin fan header

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Fan header pinouts

Pin: 1
Three-wire fan support: Ground

Pin: 2
Three-wire fan support: 12 V

Pin: 3
Three-wire fan support: FAN_TACH

Pin: N/A
Three-wire fan support: N/A

Note: When connecting a three-wire fan to a four-pin fan header, the fan is always on; there is no fan control

Usually 3 wire is tachometer for rpm and the 4th wire would be to control speed.

If your current BIOS does not show fan speeds your OpSys can, whether by 3rd part app or in Linux through lm_sensors or some versions of hwinfo.