There are 2 4pin connecters (CPU fan and CHA_Fan1) and 4 3pin connecters (CHA_Fan2, CHA_Fan3, NB_Fan and PWR_Fan)
What differentiate the 4 3pin fans? I noticed any fans plugged in CHA_FAN2 and CHA_FAN3 don't move very fast while the fan plugged in NB_FAN goes at maximum. I plugged in 3 identical fans (CM Blademaster) which are 4pin PWM fans, so I expected them to run the same.
Is the NB_FAN not controlled like the other 2 fan headers?
Here is the quick answer: 4-pin fan headers have the extra pin for speed control, as determined by software in response to measured heat.
Three pin fan headers do not regulate speed (unless you have the odd voltage controlled speed as some Gigabyte boards do)
Download HWMonitor and you can watch the fans speeds.