add a column to the cart for the discount percentage. use the decimal value, so 0.1 for 10%, etc. name the column discountPercentage
on the calculations tab, create a calculation named discountAmount as:
abs([Quantity] >= 3)?([Price] * [Quantity]) * [discountPercentage]:0
on the discounts tab, create the discount as:
trigger:
"Subtotal for any column" if the subtotal of column "discountAmount" as ">" a value of "0"
Calculation:
"Based on column subtotal"
subtotal for column "discountAmount" "plus" "0"