(3) Without prejudice to the power of extension conferred under this Act, where a certificate under this section is in force in respect of a ship and the certificate was issued for a shorter period than is allowed under this section, the Minister or any person authorised by the Minister for the purpose may, if satisfied, on receipt of declarations of survey in respect of the ship that it is proper to do so, grant an extension of the certificate for a period not exceeding one year and not exceeding, together with the period for which it has been previously extended under this subsection, the longest period for which it may have been issued under this Act.