A certificate of registration shall, subject to the provisions of this Part of this Act, remain in force for a period of three years from the date of issue :
Provided that, where an application for the renewal of such a certificate has been made, that certificate shall remain in force pending the final decision of the sheriff on such application for a period not exceeding three months from the date on which the certificate would otherwise have expired and, if the sheriff thinks fit, for a further period not exceeding three months.