I have installed a Noctua NH-D14 onto a Gigabyte GA-X58A-UD3R motherboard and used the y-split cable to group both fans to one power cable, as suggested. However, the motherboard's CPU FAN power socket has 4-prongs and the Noctua plug only has 3 holes. The Noctua plug does fit on the 4-prong socket on the mobo with one prong still sticking out, but that can't be right... Some of the other fan power sockets on the mobo have 3-prongs that would fit the Noctua plug (like the PWR FAN sockets). Do I leave it on the CPU FAN with one prong sticking out? Or switch it to a PWR FAN socket with the correct 3 prongs? I know others have had this board and fan, so I think there's a way (hoping). Thanks for any assistance.
A 4-pin fan port on a mobo can always accept a 3-pin fan connection and it will work. Exactly how it will work may vary, according to how the BIOS was designed.
When you plug in this way your 3-pin Noctua will receive the proper Ground and +12 VDC connection to power it at full speed, and the third (yellow) wire will deliver the fan's speed signal back to the mobo to be monitored. No problems there. the only "problem" is the question of whether or not your mobo will try to control the fan's speed (reduce it when appropriate). A 4-pin fan gets those same 3 signals on the same pins as a 3-pin, PLUS it gets a fourth (PWM) signal. The fan has a small controller built into its case and it uses the PWM signal to control how much current from the supply actually does flow through the fan motor, thus achieving speed control. A 3-pin fan cannot do this, so it will just run at full speed all the time because its + supply is always at 12 VDC.
It is possible to have the BIOS designed to adapt to 3-pin fans, but not all do. Some merely assume they will put out signals suitable for only 4-pin fans. But some can be set manually in BIOS Setup to change to 3-pin fan control mode. In fact, I suspect a few are even smart enough to figure out which fan is connected and make this switch by themselves. Anyway, in 3-pin mode the +12 VDC supply line is VARIED (reduced) according to requirements, and that's how the 3-pin fan's speed is controlled. The basis for control in either case is that you set a temperature target, and a sensor built into the CPU itself sends a real measured temperature to the BIOS. Based on the difference between measured value and target setpoint, the control circuit alters the fan speed to achieve temperature control within the CPU. This is known as a classic Feedback Control Loop - the measured result of the controller's action is fed back to the controller and compare to setpoint; the difference is manipulated and turned into the controller's output signal to actuate a device (the fan) that will affect the measured result.