Commerce Question - Triggering Discount

Site URL: http://www.listenlearn.ca

Building an ecomm site for a client. listenlearn.ca Password: LLC.  I just purchased the advanced commerce plan for my client to take advantage of the advanced discount feature. In particular, she is looking to have an automatically triggered discount where you would add 3 items from the "t shirt" category and a $5.00 off discount would apply. Does anyone have insight on how to apply this or