Error on mean or standard deviation

#1
I have a plot which has average values on the y axis but the averages are based on only 3 samples.
Should I use the standard deviation or the error on the mean for error bars?
The error on the mean is obviously smaller and so looks more senible in terms of error bar size.

Apologies if a similar question has been posted before but I can't seem to find the answer!

Many Thanks.