I am flying to Hong Kong for a holiday, but wish to visit Mainland China for 3 days as a tourist.

Hello Helen,
Some cities (including Beijing, Shanghai, Guangzhou, Chengdu...) in Mainland China has a 72-hour visa-free policy, but this only applies to those who have a transit stay through these cities and will fly to a third country. It does not apply to passengers who travel on round trip flights e.g., Hong Kong - Beijing - Hong Kong.

If you just want to visit the Pearl River Delta region (Guangzhou, Shenzhen, Zhuhai...), you can join a local travel agency and enjoy a visa-free stay up to six days.