Actually, you MISunderstand that 4-pin fans only can be controlled.
The speed of BOTH 3-pin and 4-pin fans can be controlled, but the method is different for the two types.
For 3-pin fans, the connections to it are Ground to Pin #1, +VDC (varying) to Pin #2, and Speed signal on Pin #3. To control fan speed, the mobo header must alter the voltage on Pin #2, ranging form +12 VDC (max) to about +5 VDC. (At voltage less than 5 VDC, the fan may stall and not re-start until the voltage is increased.) This method is called "Voltage Control Mode" or sometimes "DC Mode". The fan speed signal is a series of pulses (2 per revolution) generated inside the motor and sent back on Pin #3 to the mobo for counting and display. Interestingly, this signal is NOT used for actual control of the speed.
4-pin fans work differently. They still use almost the same signals on those first 3 pins, but with one important difference. Pin #2 always has +12 VDC on it, never reduced. Then the new Pin #4 has the PWM signal. Inside the fan a small chip uses that PWM signal to control the flow of current through the motor from the +12 VDC supply, thus manipulating speed. This method is called "PWM Mode".
The connectors of the two fan types are almost the same so that either type can plug into either type of mobo male header. The signals are so similar that mixing them sort of works. Obviously, of you don't mix, each type works as intended. If you plug a 4-pin fan into a mobo header that uses Voltage Control Mode the fan receives no PWM signal and cannot modify the power supplied. BUT the supply on Pin #2 is a VARYING voltage which then gets fed to the motor windings unchanged, and the motor's speed IS under control by the mobo. But the other way - plug a 3-pin fan into a header using PWM Mode - does not give control. The motor receives a fixed +12 VDC on Pin #2, and has no way to accept or use the PWM signal on Pin #4, so the fan motor always runs full speed.
Now, here's the "trick". You can NOT tell from the count of pins on the mobo header which type of control it is using. You have to play detective with the mobo's manual. For yours, see p. 15 where the diagrams are shown for the three SYS_FAN headers that have 4 pins. It says that the signal on Pin #4 is "VCC", whereas the label for the CPU_FAN header has the label "Speed Control" on its Pin #4. This means that the CPU_FAN header IS using the PWM Control system, but the SYS_FAN headers are NOT - they are using Voltage Control Mode. In other words, although the fan headers have 4 pins to make you feel comfortable, they actually operate exactly like 3-pin Voltage Control Mode headers. Why would they do this? Well, look at what I said above about "mixed" systems. A header using Voltage Control Mode CAN control BOTH fan types. So, no matter which fan type you bought, the mobo SYS_FAN headers CAN control the speed of them with this design. (It is a problem with one specific case - IF you are trying to use a 4-pin HUB for many fans - but you are not doing this.)
So, bottom line - your 3-pin fans WILL be controlled by your existing mobo SYS_FAN headers with no problem as long as you leave the fan port configurations in BIOS Setup (manual p. 27) in their default settings: "Normal".
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