Stopover visa needed????

I am travelling to Beijing from Hanoi and intending to get a 72 hour tourist free transit there before heading to Hong Kong. My flight between Hanoi and Beijing stops over in Guangzhou airport for three hours, will I need a visa for this??? Also my flight between Beijing and HK stops over in Shanghai. I am worried as including these stopovers I will stay for over 72 hours, although my time in Beijing will be less than 72 hours. Please help! :)