68–95–99.7 Rule

is a shorthand used to remember the percentage of values that lie within an interval estimate in a Normal Distribution: 68%, 95%, and 99.7%

three-sigma rule of thumb (or 3σ rule) expresses a heuristic that nearly all values are taken to lie within three standard deviations of the mean, and thus it is empirically useful to treat 99.7% probability as near certainty