I've noticed that when we play 3 at the back (or, if you like, with 'wing backs) we always appear to be undone by a long diagonal ball, either in the air or on the ground, seemingly because the wide players offer little in defensive discipline.
Whether this is a tictac that the other clubs have worked out or just happens during the run of play, we cannot handle it.
Liverpool exploited it brilliantly imo.
That said, we still appear to be undone by a straight long ball too.