One of the imported cases involves a 19-year-old woman who flew in from the United Kingdom, while another is a 47-year-old woman from Indonesia. Both of them were confirmed to have the virus while staying at quarantine hotels.
The third patient is a 20-year-old man who arrived from Namibia.
All of the patients were found to be carrying a L452R mutant strain.
Officials also reported under five preliminary positive cases.
Hong Kong has recorded 11,920 Covid-19 infections so far.