I had to replaced my old Rear Fan in my PC case. The new Rear Fan has 3-pins and the old one had 4-pins (PWM). The Rear Fan is plugged into the 4-pin rear header of my Intel DZ77RE-75K Mobo (with latest Bios 0066). The new Rear Fan is running however the Fan Speed is not reported anymore in the Intel Desktop Utilities (version v3.2.8.089). The speed old 4-pin Rear Fan was reported. It seems to me that the new Rear Fan is running at a constant speed and is not anymore controlled by the Intel DZ77RE-75K Mobo.
In the Intel DZ77RE-75K Mobo manual (March 2014 version), on page 12, there is a reference to "fan speed control using voltage control on Intel DZ77RE-75K Mobo (4-pin fan headers front, rear, and auxiliary) with selectable support in BIOS for 3-wire fans". I am assuming this would be a Voltage Modulation control of the new 3-pin Rear Fan speed based on the temperature detected by the Intel DZ77RE-75K Mobo. This is an option available on my old ASUS Mobo.
However, I could not find in the BIOS the option for selectable support for 3-wire fans.
I verified the speed of a 3-wire fan can be increased or decreased by modifying the manual duty cycle. You may try with a third party application and the fan control mode set to auto.
There is no capability provided in BIOS for selecting support for 3-wire fans - and indeed none is necessary. With the exception of the Processor (CPU) fan header -- which supports only a 4-pin, PWM-controlled fan -- all other fan headers on the 7 and 8 Series Intel® Desktop Boards utilize a hardware circuit that automatically detects, at power-on, whether a 4-pin PWM-controlled fan is present or not. If a 4-pin PWM-controlled fan is detected, a full 12V output is supplied to the fan and it implements its own fan speed control based upon the PWM control signal. If a 2- or 3-pin fan is detected (or a non-compliant 4-pin fan; alas there are some out there), on the other hand, a voltage-variance control circuit is automatically enabled and the fan speed is controlled by varying the voltage between 0 and 12V.
OK, so your next question is "Why isn't the fan speed being reported?" There are a number of possibilities:
Hope this helps you get things working properly...