My understanding from this page: http://www.octopuscards.com/consumer...s/en/index.jsp
Is that if you get a ticket which is Two Airport Express segments plus three days of MTR then the Airport Express segments are valid for 180 days and the 3 days of MTR is 3 calendar days including the day of first use on the MTR (implcitly excluding the Airport Express). I believe that "calendar days" in fact run from 3am (or sometime after the MTR closes down - so catching a train at 1am on a Saturday would count as Friday for this purpose).

My expectation therefore would be that if you use the airport express plus a taxi or shuttle bus to get to your hotel on the Friday evening and do not use the MTR until Saturday morning then the ticket would cover your entire stay.