I find the CSP v3 brake pedal too stiff, I was wondering if I can use the Clutch pedal as brake pedal.
You can bind in game what ever you want of course. If you want to bind the "b" key of your keyboard you can also use that for braking.
But I would not prefer the Clutch on a v3 as a brake pedal. Not really a liniear feel because of the clutch feel mechanism. Plus I don't know how long you have the v3 pedals, but give it sometime to get used to the firmer feel. Don't focus on travel in the pedal but more a consistant force. You can always lower the BRF value a bit.
have you set the brake to the softest setting? a load cell brake is a bit harder than one with potentiometer