I have these ARCTIC F12 PWM CO that have a 4pin PWM connector. I also am running 8 of them. I need a fan controller to run all of them and will most likely have to use Y splitters to connect them all. However, it seems all the fan controllers I have found only have 3pin connectors. So, that would mean I would need a cable that had 2 4pin PWM connectors on one side and a 3pin connector on the other side. Does such thing exist?
More about :make double 4pin pwm 3pin splitter cables
3 pin fans control with voltage while 4 pin control with a duty cycle.
Now you have to watch out with splitting 3 pin fans as it places more load on the board.
This limit applies to 4 pin fans as well, but they take 12 volts all the time and the duty cycle tells the fan's motor how fast to go.
The adapter listed by the above poster is good because it prevents load on the board by making the power supply carry the load. Just ensure you only use 4 pin fans with that kind of adapter as 3 pin fans with it will run full speed all the time.
pwm fans(4-pin) will run on voltage control(3-pin) based fan controllers the extra pin just hangs off. It may not control quiet as well, but should work.
If you have multiple PWM headers, the adapter(a few of them) listed above may be a good way. Just remember you will have lots of wire to hide and only 1 of fans per adapter will report speed to the board.