American Studying in Hong Kong-Can I still do the 144 Hour or 72 Hr. Trip to Shanghai?

I currently have a visa to study in Hong Kong and I am wondering if I were to fly from Hong Kong to Shanghai and then back to Hong Kong, would I still be eligible for the 72 or 144 hour trips? It is just kind of confusing because it says that you must travel to a third city so I'm not sure if it works for round trips too. Thanks!