Doubt

Renan, unfortunately, the only portion that is eligible for 144 hour visa free transit is your first itinerary:

Los Angeles - Shanghai - Seoul

This is perfectly eligible and you are fine with different airlines or separate tickets

Anyway, your two other itineraries are plain round trips and they cannot be considered as a transit. They require a visa. Of course, if you want to invest probably a bit higher amount of money for some different itineraries, you could perform even the mentioned trips. For example, for both Beijing And Changsha, you can use something like this:

Seoul - Beijing - Osaka - Seoul
Seoul - Changsha - Osaka - Seoul

Using the above mentioned itineraries would allow you to enjoy 144 hour visa free transit in Beijing and 72 hour visa free transit in Changsha.

My suggestion is that you calculate the entire price of the above mentioned flights and the price of issuing a Chinese visa. Maybe it would be more feasible to obtain a Chinese visa an to explore Beijing and Changsha (easily reachable by high speed train from Beijing) in one go. You will decide, but just to have an opinion of some other possibility.

By the way, I assume that you are a Brazilian citizen because in this case you are eligible for 72 and 144 hour visa free transits.

Hello!
I thought about change my route and doing the following:

LAX> SHA
Shanghai 3 days

SHA> SEL
Seoul 3 days

SEL> PEK
Beijing 3 days

PEK> HND
Tokyo - 3 days

OSA> SHA> LAX

Using this flights I can go without Chinese visa, right?

Thanks

That's correct.