Hey peruvian.

I have a feeling you want to find the value of n so that three standard deviations (or standard errors) are in some particular interval.

For this you need to fix a standard error from your sample (or a standard deviation that is assumed for your population) and solve for n where se/SQRT(n) < 1/3 or 3*se/SQRT(n) < 1.

A value of 3 gives roughly 99.7% confidence which is what you are looking for as a rough sketch.