Need help with the Math and placement

I have read the directions, but I am missing something.

I think I need step by step instructions on this one.

Here is the issue: There is a minimum purchase of 3 items – Any combination of 3 items in the store. Customers can purchase 3 different items, the total will still be $20.

Each item after that is $8 plus shipping and handling.

If I have done my math correctly, it should go like this:
3 items will be $20 + $14 s/h = $34.00
4 items will be $28 + $14 s/h = $42.00
5 items will be $36 + $20 s/h = $56.00
6 items will be $44 + $20 s/h = $64.00

Please, how do I get WooCommerce to understand that?