Mont. Code Ann. § 15-30-2607

An application for revision may be filed with the department by a taxpayer within 3 years from the last day prescribed for filing the return as provided in 15-30-2605 (3), regardless of whether the return was filed on or after the last day prescribed for filing. If the department has revised a return pursuant to 15-30-2605 (3), the taxpayer may revise the same return until the liability for that tax year is finally determined. If the taxpayer is not satisfied with the action taken by the department, the taxpayer may appeal to the Montana tax appeal board.