How can i set up a cart promo rule :
If a customer buy 3 items from products without special price, then he/she get a discount 15% only for items in cart which haven't special price?
This should be the solution.
Conditions:
Check when there is more than three quantity in the cart
For each row item that has more than three quantity and no special price.
Actions:
Fill in discount amount 15
Condition for this price rule; special price is not greater than zero