Idaho Const. art. XVIII, § 10

The board of county commissioners shall consist of three members. Their terms of office shall be as follows: At the general election of 1936 two members shall be elected for a term of two years and one member for a term of four years; at each biennial election thereafter one member shall be elected for a term of two years and one for a term of four years. The legislature shall enact the necessary measures to put this provision into effect and in so doing shall allot such four year term to each commissioner’s election district or like subdivision of the county which may be provided by law, in rotation.