According to the empirical rule, if the data form a "bell-shaped" normal distribution, approximately percent of the observations will be contained within 3 standard deviations around the mean.

According to the empirical rule, if the data form a "bell-shaped" normal distribution, approximately 99.7 percent of the observations will be contained within 3 standard deviations around the mean.  

To add, a bell curve is used to describe the mathematical concept called normal distribution, sometimes referred to as Gaussian distribution.