The 3 column layout splits a row on an advanced page into three visible columns. Under the layout option, three setups are available. They are Left Large, Center Large, and Right Large.
In the Left Large layout option, the left visible column will span two columns, while the center and right visible columns will only span one column.
The Center Large means that the left visible column spans one column, the center visible column is the one that spans two columns, and finally the right visible column will only span one column.
The Right Large is the final option, and it means that the right visible column will span two columns while the other two will span only one column. As with other situations, adding a widget will override what is in the rich text right above it.