The Mayor may at any time transfer any unencumbered appropriation balance or portion thereof between general classifications of expenditures within an office, department or agency, subject to the approval of the State Tax Commissioner. At the request of the Mayor and within the last three months of the budget year, Council may by resolution transfer any unencumbered appropriation balance or portion thereof from one office, department or agency to another, subject to provisions of general law. No transfer, however, shall be made from the appropriations required by subsections (1), (2) and (3) of Section 12.18 of this Charter.