Test matrix
A = rand(2,3,3);
Solution #1
D = reshape(permute(A, [1 3 2]), [6 3])
Solution #2
B = mat2cell(A, 2, 3, [1 1 1]); % slice into 3 cells (along dim 3)
C = squeeze(B); % flatten from dim 3 to dim 1
D = vertcat(C{:}); % vertically append the three matrices
