The tax for the payment of the outstanding bonds levied by the quorum court of the county issuing the refunding bonds shall continue for the payment of the refunding bonds and certificates of indebtedness. If the tax proves insufficient to meet the maturities of the refunding bonds with interest, it shall be the duty of the quorum court of the county to increase the levy of taxes, but not beyond three (3) mills upon the dollar of the assessed valuation existing at the time of the levy.