OK, so you need to know the ID of the product and the price of the product this discount applies to before creating the rule.
For the sake of example, lets say the ID is 22 and the Price is $10.00. so for the third item the discount will be $5.00
In the eCart object, go to the Discounts tab to create the discount.
For the trigger use:
Presence of item in cart:
if item '22' 'exists' in the cart
Quantity Of Specific Item In cart:
If the quantity of Item '22' in the cart is '=' a value of '3'
the use a flat rate calculation of '5.00'