Mean on 3d Array in 3rd dimension and 3d arrays indexing in for loops

Hi,

My final goal is to make a 3D convolution. I have one big 3D array (1000x1000x3) and one 3D small one (5x5x3) that I used to calculate the average value of value of 5x5x3 tiles in the big array.

My current strategy was to use a 2D convolution with my small 7x7 matrix and than make a mean in the 3rd direction. I block on how to make a mean in the 3rd direction. How do I do that?

Also, when I input a 3d array in a for loop, I know that it ''becomes'' a 2D array in the loop but I'm not in what sense does he slice the 3D array to make 2D arrays.

Also, keep in mind that I need this code to run as fast as possible.

Thank you for the help.

RMT