From a Chimese Numerology website:

The number "3" is used many times in Chinese culture. The only reason that I can think of is because it is mentioned in the I Ching, pronounced E Ching. It stands for Heaven, Earth and Man. When we bow to offer incense, it is always done three times. In bonsai and ikebana, the patterns follow an irregular triangle to represent the trinity of heaven, earth and man.

By Don Gee
www.chcp.org


Might have something to do with it.