In a case where a taxpayer has children both under (1) and (2), he shall first deduct allowance for the children under (1), followed by the children under (2). Except where the taxpayer has 3 or more living children under (1), he shall not deduct allowances for children under (2). If he has less than 3 children under (1), he may deduct allowances for the children under (2); however, the total number of children shall not exceed 3.