Pretty easy. What you need to learn is how MATLAB stores numbers in an array, how to reshape things so that you get what you want.
For example, suppose you reshaped the array to be 3x10x10? Essentially, then each column of the new result will be the three numbers that you want to average. The idea is to bring the numbers you want to associate into ONE column, with each set as one column.
Then you would take the mean of each solumn of that array. The result would be 1x10x10, and we will have averaged the elements together that you wanted averaged.
Getting close. Now all you need do is reshape that result to be 10x10. Or, you could learn to use the squeeze function, and why that function is valuable.
The point is to visualize how the data lives in your array. Then think about how to get where you want to go. So this should work:
B = squeeze(mean(reshape(A,[3,10,10]),1));
