Thanks. It's great to learn that the Airport Express Tourist 3-day is 72 hours from first MTR use, not 3 calendar days.

I had expected, thought, and hoped as much -- but then had thought http://www.mtr.com.hk/eng/train/ae_tourtickete.htm would say "the 72 hours of unlimited MTR rides (except on Airport Express) start at the time of your first MTR journey" rather than "the 3 days of unlimited MTR rides start on the day of your first MTR journey."

Will be sure to confirm before purchase, but will plan accordingly.

Thanks also for the tip re the bus -- which, as you say, wouldn't trigger the start-of-MTR effect. Will definitely consider using that trick in a pinch.