So your variable has 3 dimensions? If you ran the command size(A), you should get three numbers, something like m x n x z.
If that is the case, then you can consider your variable a stack of 2D sheets, where each sheet contains an m x n matrix. The question, then, is which sheet do you want to view? The notation is straightforward. Do the following to extract all the values in the first sheet.
tmp = A(:,:,1)
