You can use the function min to achieve this. Read about it.
% demo data using 3D matrix
A(:,:,1) = [2 4; -2 1];
A(:,:,2) = [9 13; -5 7];
A(:,:,3) = [4 4; 8 -3];
[val,idx] = min(A,[],[1 2],'linear') ; % gives min value and global index
[i,j,k] = ind2sub(size(A),idx) ; % get sub index from global index
idx gives you the global index, you can convert them into sub index using ind2sub. Read about this function.
