3. Every such public officer shall send to the Minister of Revenue, with the return mentioned in section 2, twenty per cent of the balance, over three thousand dollars, of the net amount of the fees and salary received by him for the period covered by such return, after deducting the necessary expenses of the office, which expenses so deducted shall, in no case, for the purposes of this act, be more than one-quarter of the total amount of fees and salary received.