Discount Promotion for Orders with Multiple Item Categories

Our sales department has requested a promotion to encourage the sale of items across multiple categories. The gist of it is that if an order contains items from 3 predefined categories, then all items in those categories receive a 5% discount. (The goal is to encourage the purchase all the categories representing  a product family, and not just a part of it.) The promotional item discount seems close (with buy/get) but I don't see how it could be used to discount the items in the buy list.

Any assistance or guidance is greatly appreciated.

Christopher