[Events 2.0] Include conditional/automated promo codes

I built a 3 day event in the new version.  Each day has a charge and you can sign up for each day as stand alone lecture. But if the customer signs up for 2 days, they get the 3rd day free.  I cannot figure out a way to define the promo code to only work if they spend x amount first.  I am finding that I have to manually sign them up for the 3rd day but now I cannot zero out the charge.  This will mess up my event accounting.  Thanks.