I have never used one myself, but based on all I've read and all I've been told, it's a 72 hours, not three individual days.

But keep in mind that the unlimited 72 hours apply to MTR travel only. It doesn't start with your first Airport Express use, doesn't start with your first purchase of other goods/service with that $20; it shouldn't even start with your first free AE-MTR transfer. But it starts with the first actual MTR ride.

Countdown? The most lively and crowded place is Times Square in Causeway Bay.