'\n' is new line. I think you want '\t' for tabs at the end of each cell and a new line at the end of the row. By adding an inner loop to go though each element in the cell, you can make the code even less sensitive to how many elements are in each cell.
fileID = fopen('result.txt','w');
[nrows,ncols] = size(pop3);
for row = 1:nrows
for col = 1:ncols
formatSpec = '%d '; % Puts a space between elements in the same cell
for idx = 1:numel(pop3{row,col})
fprintf(fileID,formatSpec,pop3{row,col}(idx));
end
fprintf(fileID,'\t'); % Add a tab at the end of each cell
end
fprintf(fileID,'\n'); % Add a new line at the end of a row
end
