I already have an octopus card. How can it be useful for us?

Did you purchase the Standard Octopus Card? If yes, you are not available for 3 days unlimited metro. Only Airport Express Travel Pass holders can enjoy this.
From the airport, you can take bus no.A21 from Airport (Ground Transportation Centre) to Middle Road, Nathan Road. Then walk north along Nathan Road around 50 yards. The duration is about 70 mins. And you can use the Octopus Card to purchase the ticket.