If it's for a single product, just use the Discount tab of the product data. You can enter the quantity and corresponding price in there. For example, you'd make 3 products cost 3.3333 so the total is $10.00.
If you instead want the discount to be its own line item on the order, you could use an extension like
Category & Product-Based Fee/Discount. It can create fees or discounts based on a variety of criteria, including the quantity of specific products. In this case, you'd set up a discount for that product like this:
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CHARGE #1
Title: Product X Discount
Group: A
Charge Type: Quantity
Charge Field: 3-5 = -0.50, 6-8 = -1.00, etc.
Rule: Product is Product X
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Feel free to take a look at the screenshots and demo site, and if you're interested let me know at
www.getclearthinking.com/contact if you have any further questions.