Designing barbettes for future turrets

How could a ship be designed so twin turrets might later become triples?

Make the ship and barbette wide enough from the outset; for the same calibre, the barbette needs to be substantially wider than a normal twin mounting.

A twin mounting cannot simply become a triple later unless the hull and barbette were designed with that larger installation in mind. For the same calibre, the barbette would need to be roughly 30-40 percent wider. The wider opening also requires the hull itself to have enough beam and structural allowance from the outset. Early structural choices therefore constrain later modernization.