Sorry, but we don't have any built-in way of doing quite what you describe. This help page explains our discount settings in detail:
You can offer a single flat amount or percentage off the buyer's pre-tax/-shipping item total when Min. Item Qty. = 3 or more, but there's no way to make that quantity threshold only count certain products and not others.
You can create a series of product-specific discounts that all use the same Discount Code (note they must each have a unique Discount Name), so the buyer only needs to enter that code once to markdown the original price of all items in their cart which have a matching discount. However, there's no way to require a minimum number of those items before the discount can be applied (in this case, each item discount's Min. Item Qty. threshold only counts the quantity of that particular item).