For the first 3 years, the interest will not be changed because of the 3/1 ARM policy. The interest will stay at 4.15%. The monthly payments will be based on the 4.15% and 20 year period. The interest is then converted to effective monthly rate by dividing by 12 and payments will computed using the formula
A = Pi (i + 1)^n / [(1 + i)^n - 1]
