Istanbul - Hong Kong - Xiamen - Hong Kong - Istanbul

Hi,
i am a german Citizen and i want to fly with Turkish Airlines from Istanbul to Hong Kong and same day with Cathay Airlines to Xiamen. Want to stay 2 days in Xiamen and fly back with Cathay Airlines to Hong Kong. Want to stay 1 day in Hong Kong and fly with Turkish Airlines back to Istanbul.
Can I use 72 Hours Free Transit to stay in Xiamen or do I need to apply a Tourist Visa for China?
If 72 Hours Free Transit is ok, where i have to apply it? On Check-In in Istanbul (TK) or on Check-In in Hong Kong (CA) before flight to Xiamen?
Thanks for your answers.