Hello. Do we qualify for the 72-hr visa free entry?

I'll be visiting Hong Kong from US for about three weeks. I plan to visit Singapore and other countries with my kids while in Hong Kong. Can I book a flight from HK - Guangzhou- Singapore to qualify for the 72 hr free entry? And does my return flight from Singapore have to come back through Guangzhou, or can I go directly from Singapore back to Hong Kong? Thank you.