US to Shanghai (72 hrs) then to Singapore 7 days, back to Shanghai (72 hrs) then fly back to US.

Hi, I am a US passport holder, planing to fly to PVG and stay there for 72 hours and then fly to Singapore and stay there for 7 days and then fly back to the city to stay for another 72 hours and fly back to US. Do I qualify for free transit? Will I Need a visa?