Top 3 products per category

For each category, find the top 3 products by revenue from completed orders.

• revenue is the sum of quantity * unit_price
• products that tie on revenue share a rank. Include every product whose rank is 3 or better, so a category can return more than 3 products. For example revenues 500, 300, 300, 200 give ranks 1, 2, 2, 4, so the first three come back and 200 doesn't.
• products with no completed sales aren't ranked at all