(3) The Municipal Revolving Fund shall be distributed annually, during the month of October, to all municipalities on a population basis, using the latest federal census in computations, taking into consideration the entire population of each municipality in the state, and taking into consideration municipalities that have been incorporated since the last federal census, or will be incorporated before the next federal census, in which case the population shall be the official count used in procuring the charter of incorporation, and also taking into consideration any county seat that is not an incorporated municipality as though the county seat were an incorporated municipality. In making distribution to an unincorporated county seat, however, the funds computed to be due to the county seat shall be paid to the county treasury in which the county seat is located. Funds made available to municipalities under the provisions of this section may be used for any lawful municipal purpose, except that where funds are made available by reason of the location of an unincorporated county seat in any county, the board of supervisors in that county shall use the funds for road, bridge and street construction or maintenance.