3 seems to be the most sacred number in Christianity-being the number of the trinity. As karstdejong said 7 is mentioned in the Bible quite a lot (it's also the number of heavens in Islam culture), as indeed was 40, 12 of course has some significance too.
four was also inportant as it reperesented the temporal virtures, fsix as it was the numbers of days of creation and then you can multiply and add and so on...
See Unberto Eco in Foucaut's Pendulum for a wonderful micky take on numerology.