The 2nd wire is what powers the fans and can allow you to control the fan speed.
Connect multiple fans to motherboard.
The last pin just won t be used.
However if you have a particularly large pc case that you plan on filling with multiple rgb fans each requiring its own.
The cabled labled to rgb hub will plug into the small 6 port corsair rgb fan led hub that comes with the multi pack of fans.
This wire can either be plugged into a normal fan header on your motherboard or preferably into a corsair commander pro.
Finally you can do molex straight to the fan which i don t have much experience with but iirc you need a hardware volt adapter small cable thing to avoid running at full speed at all times.
Well that depends if that fan splitters have 4 pins that means its pwm which the fan speed ia controllable you can use 4 pin splitters in 3 pin applications.
You can also get pwm fan controllers powered through molex sata power or the like where you can connect several fans and control them via software.
In fact there are a number of ways to do so.