I transit in Guangzhou and I have third country confirm ticket with seat how can I get the permit to stay for 72 hour

Before the trip, you need to contact the airline company you use and tell them you want to get the 72-hour free visa policy. Then they will helpp you to apply for it.
When you reached the GZ airport, you can just show the custom your tickets and the passport and then you can get out of the airport.