An image functions as a 3 dimensional matrix and in order to index a 3x3 matrix out of an image, you can do the following:
im = imread('peppers.png');
patch = im(1:3,1:3,1:3); %indexes first three rows with first three columns and all three RGB values
center = patch(2,2,:); %this returns the center of each layer.
imshow(patch,'InitialMagnification','fit'); %shows the 9 pixels magnified
If you can describe what you mean by "3x3 patch" regarding an image in more detail, it could help me provide some more relevant code for your use case.
