72 hours transfer

Actually, it is not visa free. You can rely on 5 day Shenzhen visa on arrival when crossing the border between Hong Kong and mainland China and use it for your 3 days stay in Shenzhen area. This visa is limited to just administrative area (within the boundaries) of Shenzhen city. After your stay in Shenzhen you can return to Hong Kong by taking any border crossing and any mean of transport including a high speed train.

If you decide for this option, it is recommended that you take Luohu (Lo Wu) border crossing, but have in mind that visa office is open just until 16:30. Also, a visa price is quite steep for US citizens because the price is 956 CNY. Also, you should bring with you a negative PCR or antigen test not older than 48 hours.