Because there must be one thing in each row, we only need to consider the arrangement of columns. The first thing has six columns to choose from, so there are six possibilities, while the second thing returns five and the third thing returns four. So if we don't consider whether these three things are the same, then a * * * will have 6x5x4= 120 permutations. If you think about it, all three should be arranged on this basis, that is, 6x4x.