planning a short visit to Chengdu

Hi, I,m planning a trip from Barcelona to Hong-Hong and from there I also would like to visit the region of CHENGDU or GUILIN for 2-3 days maximum, by taking a domestic flight and after this 2-3 days stay going back to Hong Kong where I will be flying back to my country. Can I use this service of 72h transit Visa-free?

and if for being eligible to use this option, I should first visit Chengdu or Guilin before arriving to HK airport? BARCELONA-CHENGDU-HK or it could be before leaving HK, that means: HK-CHENGDU-HK-BARCELONA..

Thanks in advance for your helpful answer. Regards,