With your calculation, you are just averaging your remaining interest expense over 3 years.

Don’t forget that you are paying interest on a declining balance each month.

If you go to an amortization website that shows a breakdown you will see exactly how much your interest expense is.

The interest that you paid this month is much more than the interest that you will be paying next year.
Your payment is the same so more will go to principal.

Does this make sense ?
Each month your interest portion of your payment is based on the principal balance for the previous month.