Would I be breaking the 72-hour visa transit rule by doing this?

Hello,
I am currently working in South Korea right now, and my mother and brother will be visiting in July. To see as much as possible, I am pondering flying from Seoul to Shanghai, staying in Shanghai for 2 or 3 nights, then flying to Osaka from Shanghai. And also, would I be breaking it if I fly with different airlines (for example fly with China Eastern from Seoul to Shanghai, then fly Peach from Shanghai to Osaka). Any feedback would be greatly appreciated.