This is from fractal design support
Yes, it's posssible to use y cable on the PWM headers. You can connect plenty of fans as they get power from the SATA power (and the motherboard).
This does however not apply on the 3-pin fans.
(4 pin pwm) Yes, they get 12v but they are regulated by the motherboard PWM signal.
I'm not sure if the fan hub controls the voltage of the 3 pins by the pwm, you will have to see. I'm going to use the Corsair ml140 on the 4 pins with pwm and y cables
Edit. Yes the 3 pins are voltage regulated by the pwm signal so fractal design say and also best to use the same fans if you are doing it