In those researches of sample test, especially in the works of experimental data counting and statistics, Rule of Three has been applied widely. But few people understand its origin really , and there are little research work have been down on the theory's reasoning. It has be regard as a general rule. Four different ways. Binomial distribution, Poisson distribution, Louis distribution and Bayesian approach are used,Which are. Then the rule is verified and companed through real investigation data, and four tenable conditions of the rule are given.