Yes. Expand the delivery option in your plan, tick Offer discount, then tick Change the discount after a set number of payments. Set Change discount after to the number of payments the opening discount should last, and Change discount to to the value that takes over. So 20% for the first three deliveries and 10% from then on. What 0 means depends on the discount type: with Percentage off and Amount off it ends the discount, with Fixed price it sets the renewal price to 0.00. The option badge shows both halves, and the same wording reaches the product page as a chip under the option.
One detail worth knowing, and it matters. Subscriptions that renew through Mollie step through the discount the same way subscriptions on Shopify billing do: the first order, the one paid at checkout, counts as payment one, and the price changes once you reach the number of payments you set, on either engine. "Lower or raise the discount after a number of payments" has the detail.
Stuck on any of this? Open the chat in the bottom right corner of the app and we will help you set it up.