3 Products for a flat rate

[53903]

Hi i'm looking for a contribution:

 

If my customers buy 3 products in one category they will get a flat price.

Example:

 

categorie -> shoes

product1 -> nike $40

product2 -> adidas $50

product3 -> reebok $60

 

normaly it will be $150 total but now i want if you buy 3 that it will be $125 for example

can somebody help me

 

Thanks

53903