Ability to apply multi-year stepped discounts prior to full-price subscription period

Hi -

Here is our requirement-

New customer signs a 5-year contract. They agree to pay $200 per month for 5 years, but in year 1 we give a heavy discount ~75% and second year mid-level discount ~50% off the price they agreed to pay in year 3.

in year 3, 4 and 5 customer will pay the full price (i.e. $200 per month)

How can we do this in Siebel? Appreciate any help

Thanks