If I am not forgetting something, you should do it within ebay from the console. Let’s check discount sales (don’t know the exact name of that page in English), there should me something like: buy two = 1$ discount, buy three = 3$ discount and so on. Make sure to open one discount sale only for that specific listing. If it lets you add only 1 discount rate, start more discounts for each discount rate you would like to start. This functions works differently from each ebay site, so I am not able to ensure it will work also there, let’s give a try…
Hope this will help you.