When the commissioners of the existing highway district order the establishment of a new highway district, they shall have the duty to name the new highway district and to divide the new highway district into three (3) subdistricts, as nearly equal in population, area and mileage as practicable, to be known as highway commissioners subdistricts one, two and three. Subdistricts may be revised or modified as changes in conditions demand. Not more than one (1) of the highway district commissioners shall be an elector of the same highway subdistrict.