Need three entries into china over a 2 and half week period.

I have applied for a double entry visa but need three entries. My itenerary is as follows:

1- Arrive to Shanghai from London
2 - The following day fly to Tokyo
3 - Fly back to Shanghai from Tokyo on Monday
4 - Fly to Seoul on Wednesday
5 - fly from Seoul back to Shanghai

Is it possible to use the 72 hour free transit and not use one of my entries when I arrive back from Tokyo as its only 48 hours?

Thanks,