Your calculator is correct.
. . You have an extra factor in your formula . . .
The money that you save over the three years will form the deposit on a home.
Assuming that over the three years you salary does not increase,
that you are going to save 15% of your gross monthly income at the start of each month,
and that you earn 6% interest compounded monthly,
determine the total amount you will save in this time.
. . . .?
This is an Annuity . . .
The formula is: .
. . where: .
Assuming that the gross monthly salary is $5,000 (you didn't mention it),
. . the periodic deposit is: .
The periodic interest rate is: .
The number of months is: .