Cart Rule scenario

[ThatDesignGroup]

Just a quick question.

We currently have a category, in which we will be selling 3 shirts for $50. This would also pertain to orders of 6, 9 (all multiples of 3). 6 for $100, 9 for $150 etc.

Just not quite sure how i'd currently set this up. I have it set up as 2 separate rules for 3 and 6 quantities at the moment, but I can't seem to figure out how to get the discount to only apply to a set number of shirts in the user's cart. I can only apply to specific products (but they're all in that category), or the whole order, so it doesn't quite help.

Any assistance is greatly appreciated.