The Octopus travel card is a prepay card you can get anywhere. It is the only way to travel on the MTR. If you are there for 3 days and expect to do a lot of traveling you should get good use out of a three day Travel Card with a return trip to the airport on it. The Train from the Airport is great and is the fastest most comfortable way to travel. The staff are friendly anyway so you should have no problems.
Don't be afraid to buy your Octopus Card as you come out of customs and immigration - it is no more expensive than anywhere else.
If your arriving from the Airport