What is the best way to arrange the 72 hour visa free transit?

I'm a little confused about the rules about the destinations.
For example, if you fly from Dallas to Shanghai and stay around 2 days then fly to Seoul for 2 days then back to Dallas, does that meet the requirements? And what about layovers? Let's say that I fly from Dallas but it has a layover in Seoul and then goes straight to Shanghai, can I not fly back to South Korea on the way back even though South Korea was just a layover on my flight from Dallas to Shanghai? Sorry if it's confusing. I guess maybe a better way to ask my question is which is less of a hassle when picking the "3rd" country: Hong Kong or South Korea? Is it easier to fly from Shanghai to Hong Kong then back to America than from Shanghai to Seoul then back to America?