I feel having that extra body to deal with crosses is our biggest issue. One of the three will quite often leave it for one of the other two to deal with, which more often than not means it doesn't get dealt with. Even with a three we still seem to be experts at marking space when a ball comes in. There is a massive issue with them taking any sort of responsibilty for marking. Maybe a two would ensure a little more responsibilty in marking, but then again I doubt it.