3

3 is:
a rough approximation of π (3.1415...) and a very rough approximation of e (2.71828..) when doing quick estimates &  the number of non-collinear points needed to determine a plane and a circle.
Additionally, 3 is the first odd prime number and the second smallest prime & the 4th Fibonacci number. Last but not least, 3 is a biggie in religion. Enjoy :)