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[Inmost sun]

Let´s say I have 3 groups of independent data sets (n=3), each group of sample size N=5;
for each group I can determine mean (X1,X2,X3) and standard deviation (SD1,SD2,SD3);

my question is: How to correctly determine the standard error of means (SEM) of the mean of the 3 means (X1-X3)? I think I have to take the SD of each group into account, but by which method?