I'm interested in calculating standard deviation and have read your web site.

I hope to get your help:

If I have three groups of data A, B and C,

the mean values for A =a, B =b, and C =c,

the standard deviation(SD) for A =SD1, B =SD2, and C =SD3,

then what is the standard deviation for the average value of A, B and C, namely (A+B+C/3)?

Could you please tell me a formula to calculate their SD. If there are more groups of data?

Thank you very much.