For triplicates, the standard deviation is not a useful measure.
StddevP is used to describe the variation of the values, but this is very indirect and has a very complicated interpretation (being the square root of the average squared difference of the values to the mean of the values -- would you know what that means?!). It's simpler and better just to give the three values, or their range.
StddevS is used to estimate the a property of a distribution based on the information from a sample of such a distribution, specifically the square root of the variance. This estimate is difficult to interpret when the distribution is not normal*. Further, only 3 values provide only extremely little information about the variance of the distribution, so it's not very useful.
* it's still an estimate of the square-root of the variance of that distribution, but it's hard to understand what that value tells us about the shape of that distribution. When the distribution is normal, then we know that 67% of the values are expected to be in the range mean plus/minus one standard deviation.
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