How can I adapt a 3-pin PC fan connector to a 4-pin connector?
This solved my problem:
Cited from 3pin to 4pin fan adapter(Chinese)

Explanation:

two fast-switch triodes (2N-2907 and 13003) utilize PWM control over 12v power.
diode protects the fan from voltage higher than 12v
1K R gives a 12/1000=12mA ceiling to the current from 12v to ground, just in case 2N-2907 is damaged.
0.5K R does the same thing for 13003, limiting the max current approximately to 2.5/500=5mA

All these stuffs are easy and cheap to buy, enjoy..