How do I combine multiple repetitions to canculate standard error?

I am not sure that i am in the right forum, but i will give it a go anyhow:
I have 3 different samples from the same treatment, i run each one 2 times in triplicates,
so now i have 3(different samples)* 2 repetitions* three replicates each.
how can i do the standard error after all the averages?
i canculate first the avreges of the triplicates, then the averege repetitions for the same sample and only then the averages between the 3 sample...
whould appriciate any advice!