I would recommend considering using a 4-pin fans instead. The use of the 4-pin PWM solution helps meet acoustic targets in pedestal platforms through the baseboards ability to directly control the RPM of the fan.
Fan RPM is modulated through the use of an circuit located on the baseboard that send out a PWM control signal to the 4th pin of the connector, labeled Control.
3-pin fan are usually controlled by the thermistor located on the fan.
Thanks for your reply.
Unfortunately I have bought a few high-end Sanyo-Denki 3-pin fans already. I am confused about your statement "3-pin fan are usually controlled by the thermistor located on the fan." Is the third pin on a 3-pin fan not there solely to monitor fan speed? If so, what does it do if it finds that the fan has stopped, for example? If not, what does it do to boost the fan speed (say when temperature goes up)? "Thermistor located on the fan" - how does it control speed? Change the voltage? Is that somewhat related to the 3rd pin? I could not find any definite explanation on this subject anywhere. Perhaps you can shed some light here...