If three measurements have different errors, what is the error of the mean of the three measurements?

Question

For my astronomy coursework, I need to work out the sidereal day and the error of this value. Since I repeated the observation 3 times, I have 3 values for the sidereal day each with a different error. How would I work out the error of the mean of these values.

Answer 1

x = [(x1 + e1) + (x2 + e2) + (x3 + e3)]/3 = (x1 + x2 + x3)/3 + (e1 + e2 + e3)/3