Requirement of third destination country for Beijing 72-hr free transit policy

Warm greetings!
Our cruise will leave Inchon, Korea and arrive Tianjin on March 27, 2018 via Celebrity Millennemium for 3 days, then continuing on to Jesu Island, Korea again before heading onto Baoshan (Shanghai) as the final destination. We're US passport holders and are planning to join a 3 days tour in Beijing and stay 2 nights post-cruise in Shanghai. I have read your website regarding the 72 hrs free transit for Beijing and 144 hrs free transit for Shanghai. I'm still not sure how to interpret "the third country" policy. Since we will be leaving Korea to Tianjin then back to Korea again before arriving at Shanghai on April 1st. Does going back to Korea again count as third country? Do we need a tourist L type just for the 3 day tour in Tianjin? very confusing. Please help.