A 3-2-1 buy-down mortgage allows a borrower to reduce their interest rate during the first three years of the loan by making a one-time payment. In general, 3-2-1 buy-down loans are only accessible on primary and secondary residences; they are not available on investment properties. The 3-2-1 buydown is also not accessible with an adjustable-rate mortgage (ARM) that has shorter than 5 years.