Unique Coupon/Discount Need

Site URL: https://www.petros.com/shop

Recently moved a client's site from using WooCommerce (Wordpress) to Squarespace, but am having issues with unique pricing variants + coupon code/discounts. 

We need to have a discount that applies ONLY when three t-shirts are added to a cart ($10 off any three purchased). I've tried several options, but it doesn't seem like there is a way to do this... any tips/extensions/etc. I could do as a work around to make something like this work?