The board may divide the county into three districts, as nearly equal as possible in subjects of taxation, and may provide that triennial assessments shall be made each year, but for only one of such three districts during any one year. In order to inaugurate such system, a triennial assessment may be made for the first district during the year immediately following one in which a triennial assessment was made for the county as a whole, and a triennial assessment may be made for the second district during the second year following one in which a triennial assessment was made for the county as a whole. Whenever the board has divided the county into districts and provided for triennial assessments in each of such triennial districts as herein set forth, the board, in making and supervising assessments and valuations of property in such triennial districts shall make such assessments and valuations at a level uniform within such triennial districts but not in excess of the actual market value of any property assessed and valued and such assessments shall be deemed to be in compliance with the requirements of uniformity of taxation on the same class of subjects.