SEOUL - HONGKONG - SHANGHAI - TOKYO

I am a German citizen and heard that I do not need a visa for Hongkong if I plan to stay less than 90 days. I plan to travel from Seoul to Hongkong (stay there for 3 days), then to Shanghai (stay there for 3 days) and then to Tokyo. Can I use the 72 hour exemption policy for Shanghai, or do I need a visa because Hongkong doesn't count as a third country?