Unusual renewal of subscription products
I've not used UC Recurring Payments and Subscriptions before properly, but if each payment is a fully fledged order then I imagine you could use Conditional Actions to look at the date compared to the initial purchase date and give a discount if that is within the three years. The coding effort is then just creating a new condition using hook_ca_condition() which is not hard, and then after that setting up the pricing rules in the CA interface.