I concur with christep regarding the Airport Express Travel Pass. The unlimited rides apply only to the MTR system. Therefore, it depends on your planned itinerary whilst in Hong Kong to use the MTR system enough to at least breakeven. In most cases, it's better just to buy the Airport Express tickets and the Octopus card separately. If more than one of you are travelling together on the Airport Express, ask for the "group discount" ticket and don't buy in advance over the MTR website as the "group discount" is better. Only if you're travelling alone, buy in advance over the MTR website to take advantage of the advance purchase discount.

MTR has a kiosk immediately after custom and excise inspection in the "buffer" zone before the public arrival meet and greet hall. You can get your AE ticket and Octopus card there. There are also ATM's where you can withdraw local currency.

3 days in HK, you should get an Octopus card too. Load HK$200 into it. On departure, return the card at the MTR kiosk at the airport (there is one right at the arriving train platform) to get a full refund (less a nominal administration fee) for any unused stored value plus the HK$50 deposit. You may also keep the card as souvenir but in that case, no refund!