In today's Zazenkai, Jundo mentioned the significance of 3 (Buddha, Dharma, and Sangha). And then 9. And then up to 108. I've seen the numbers in many places (like the exercise of 108 prostrations some sects do) but I don't think I've ever understood where it really comes from or its deeper meanings. Are any of you familiar with this? Or know of any good resources or books to reference?