Custom Validation Message for numeric
Removed above code and added the custom validation function for CVV Validation 

public function validateCVV($attribute) {
    $value = $this-&gt;$attribute;
    if(!preg_match('/^[0-9]{3}$/', $value)) {
        $message = Craft::t("Oops!  Try again with a valid CVV security code on the back of your card.");
        $this-&gt;addError($attribute, $message);
    }        
}

$rules[] = ['CVC', 'validateCVV'];


Thank You.