Why triple turrets would give Vanguard twelve guns

If Britain adopted triple 15-inch turrets during the First World War, would HMS Vanguard later carry twelve 15-inch guns?

Yes in that counterfactual: reusing four stored triple mountings would give Vanguard twelve guns.

If Britain had standardized on triple 15-inch mountings, later ships would reuse that form under the counterfactual's industrial logic. HMS Vanguard was completed around existing 15-inch turrets, so replacing the historical twin inventory with triples gives her four triple mountings and twelve guns. The point is about inherited hardware rather than a claim that Vanguard was originally designed for that battery. Earlier turret-production decisions can therefore shape a much later ship's armament.