How to find inverse of concatenate

Suppose i have a matrix

r=[1 2 3]

g=[4 5 6]

b=[7 8 9]

rgb=cat(3,r,g,b)

Now how could I recover r,g,b matrix from rgb matrix

Answer 1

[Guillaume]

 r = rgb(:, :, 1);
 g = rgb(:, :, 2);
 b = rgb(:, :, 3);

[Roman Voronov]

how about programmatically, without doing each line manually?