shipping classes and quantity

Hi

I am trying to set up the following

Large class – max of 3 large items for ￡50 or cost is doubled to ￡100 for 3 to 6 items and so on

medium class – per item in basket at ￡15, so 2 medium class and the cost is doubled, currently the cart shows one cost no matter how many I add to the basket

small class – per item in basket at ￡7, so 2 small class and the cost is doubled, currently the cart shows one cost no matter how many I add to the basket

I have the above setup in one shipping method, apart from the calculation part

how do I set up to trigger large items (up to 3) and for the other classes to be multiplied on a per item in cart basis

is this done with the qty shortcode? cant find any documentation or examples to describe this

Shipping class costs – how are these linked to existing classes? tried class name but doesnt do anything

many thanks