Yes it's a normal distribution so 99% is within 3 standard deviations. My "3 units" description may be confusing. In my case the units is inches and I know that in my sample, 80% of the population's height is within 3" of the mean. I'm curious how to figure out the standard deviation from this type of data so I can apply it to other populations where I know X% of the population is within Y inches of the mean.