Three Layovers in China

Hi,

So I am a US citizen with the final destination being Bangladesh. However, I have a 3 layovers in China (Beijing, Shanghai, and Kunming). Do I need a visa just to be in the airport since I don't have one layover to my third country but THREE. The longest layover is at Beijing for like 7.5 hours.

What is the process or I'm good with my US passport and third country? And is it possible for me to leave the airport?