Standard deviation rule of thumb

I found this little link https://www.tutorialspoint.com/statistics/range_rule_of_thumb.htm. Now at school we say that 1/2 x range of repeat readings is the uncertainty. This means that an approximation is that the uncertainty is 2 x standard deviation. We only ever do 3 repeat readings. Thus the data doesn't really have statistical merit. This page https://en.wikipedia.org/wiki/Standard_deviation has a nice graph showing that roughly 95% of data lies within 2 standard deviations of the mean.