Why treaty battleships used three turrets

Why did battleships move from four main-battery turrets to three between the wars?

Treaty limits made three triple turrets an efficient way to retain firepower while allowing weight for armour, anti-aircraft weapons, and speed.

Treaty limits forced interwar designers to fit additional protection and weapons into a restricted displacement. Stronger deck armour, larger secondary batteries and more anti-aircraft firepower all competed with the desired speed. Replacing four twin turrets with three triple turrets increased the gun total from eight to nine while reducing the number of turrets. This arrangement helped accommodate the other demands on the design.