We arrive 8am on Saturday and need to be at hong kong airport by 8am Tuesday.

You mean AE Travel Pass with one/two single journeys, right? Well, it is valid for 180 days. In addition, you can ride free on MTR, Light Rail, and MTR Bus (Northwest New Territories), (except additional travel on the AE train, East Rail Line First Class, and travel to/from Lok Ma Chau or Lo Wu Stations) from Saturday to Tuesday.