C(:,2:2:end,:) %all rows, even columns, all pages
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Since you only want one element from each row, it will require a little extra work. Here is one approach:
C = num2cell(rand(4,8,5)); %sample cell
sz = size(C); %how big?
C2 = C(:,2:2:end,:); %remove every other column
idxEye = repmat(logical(eye(sz(1))),[1 1 sz(3)]); %create a logical 3d index of the diagonal
C3 = reshape(C2(idxEye),sz(1),1,sz(3)) %extract and reshape
