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?

Gassho,
Jeff