I have 3 Fractal Design GP-14 in my Define F5 case (the type included in the case), I bought one extra for intake, and they are very quiet at 450 RPM at idle and 600 RPM while gaming. I also have a Noctua NH-U14S for the CPU, always at about 200 RPM lower than the case fans.
Actually I never even hear the fans. I assume the X2 GP-12 in the Define C will be almost as quiet, even if you add a third one.
Nigel