I am travelling from London - Shanghai - Siem Reap - Thailand - Shanghai - London

I am travelling from London - Shanghai, staying in Shanghai for 2 nights, then going on to Cambodia and Thailand for 17 nights, back to Shanghai for 2 nights and then London. I have onward flights booked for each part.
Can I use the 72 hour visa waiver for entry and re-entry? Is there a rule on how long we need between?
Thanks