Matriculating = when you begin classes at that school
So let's say you got an MCAT score this year. That means you're good for the 2013 application cycle, the 2014 application cycle, and the 2015 cycle. In the case of 2015, you would be matriculating in 2016 which is three years after you took the test, so they would just barely be valid. You could not, however, apply with those same scores in 2016 since you would then be matriculating in 2017 which would be four years after you took the test.
Also, the date you take the MCAT doesn't matter. A January MCAT and a September MCAT both expire at the same time. All that matters is the year you took the test in.
There are some schools that accept scores older than three years though, but there's not many of them.