There is something special about the number 3, throughout history it has been a significant number and continues to be so today. The Egyptians built the Pyramids, the Bible talks of the 3 Wise Men and the Holy Trinity. There are 3 primary colours, 3 dimensions, 3 parts to time, we live on the third planet in our Solar System and there are 3 kingdoms on Planet Earth.
I could go on, but we would be here all day, so take a look at this page for more facts about the number 3.
The point I am trying to make is that we use the number 3 in many ways everyday without even realising that we are doing it, and in teaching it can be a very powerful weapon to have in your armour.