I'm a Mexican national traveling with a Mexican passport and one time entry china visa.

My itinerary is the following:
Mexico-Shanghai-Beijing- Staying three days in Beijing, then travel to Xi'an for three days and then to Hong Kong for three days. Will be returning back from Hong Kong to Shanghai, wishing to stay two days in Shanghai and then I have a confirmed flight/seat back to Mexico. Can I ask for the 72 hour free transit? if so, would it be in Hong Kong?