Flight from Phuket to Guangzhou -Guangzhou to Zhengzhou -Zhengzhou to Vancouver

Hi Dear,
I'm Canadian and I got one entry Visa to China.
I've bought round-trip ticket: DEPART March 26th,2018 RETURN: May 18th
I've changed my plan during my visit to China and decided to go to Thailand after I spent almost one month in China.
I'm now in Thailand and wondering if I can go back to Guangzhou, China and use my return flight to Canada?
I believe I will be eligible for the 72 hours free transit.
My return flight has one stop in China: Guangzhou to Zhengzhou - Zhengzhou to Vancouver.
Would you please tell me what should I do?