Bulk Price Question

Hi,

is there any possibility to achieve this:

customer buys three products of a category = 15% discount to total
customer buys four products of a category = 15% discount to total + normal price for the 4th item
…
customer buys six products of a category = 15% discount to total

So the discount should apply to a group of three products but should also work if there are for example 4 products in the basket.

I hope you know what I mean ??