How do I average uncertain values?

Hi all,
I would like to combine all my experimental and technical replicates into one value with a standard deviation. The easy part was calculating mean and standard deviation for each experimental replicate from the three technical replicates used. The part I find difficult is how to calculate the SD of the three experimental replicates, each with their own SD. The sample size is always 3 and the values are quite similar, so I don't believe I need any fancy weighted calculations. Could anybody help me out?

Thanks