Why moving the G3's third turret was not a simple fix

Could the G3's third turret have been moved behind the superstructure during design or construction?

Only through a major redesign, because the turret position was tied to armour length, machinery placement, and weight distribution.

The proposed move appears simple only if the turret is considered in isolation. Its original position shortened the armoured citadel and allowed the machinery to remain where it best served power and weight distribution. Shifting the turret behind the superstructure would move the ship's centre of mass and force wider rearrangement below decks. Such a redesign might converge on the separate F3 concept instead of remaining a straightforward late change to G3. A visible firing-arc problem can be locked into the deeper architecture of a capital ship.