Exempt from taxation are royalties received by:
(1) an Indian tribe with respect to on-reservation oil and gas production pursuant to a lease entered into under the Indian Mineral Leasing Act of 1938, 25 U.S.C. 396a through 396g;
(2) the United States as trustee for individual Indians; and
(3) the United States, the state of Montana, or a county or municipal government in Montana.