Fixed Discount on Cart (not line items)

I've got a situation here I want to give a discount if a customer buys three items of a certain Product Type, e.g.

1 Product: $75
2 Products: $150
3 Products: $200
4 Products: $275
5 Products: $350
6 Products: $400

Hence a discount of $25 dollar upon buying three products. I've used to Commerce Extra Rules module to match the Product Types in my rule. The problem I am facing is how to grant the fixed discount of $25 to the whole cart (instead of granting a discount on individual line items, which is problematic if the discount is not dividable by the product count in the rule, i.e. 3).

Any help is much appreciated.

Posted: Jul 24, 2012