2 stopover

It is not eligible for the 144 hours. When you have two consecutive stops in different transit zones (Shanghai and Xi'an in your case) you are limited to a total of 24 hours transit without visa in China, from scheduled arrival time in Shanghai to scheduled departure time from Xi'an. So you must calculate that time, and if it is a minute over 24 hours then you need a visa in advance.

The time zone is the same as Shanghai. Would this be okay?

That’s irrelevant. All of China is the same time zone.

I'm not sure if you rounded the layover times up or down but looks to be over by about an hour which requires the visa. The good news though is US passport is eligible to apply for 10 year multiple entry visa in advance of trip which costs the same as single entry visa

19 Hour layover
3 hour layover
3 hour internal flight time
------
25 hours > 24 hours

Thank you! Your answer was very much detailed and helpful compared to Saul response

You’re welcome. My answer was exactly correct, by the way, sorry you couldn’t understand it.