Why a three-deck ironclad would remain a rarity

In a world of two-deck ironclads, how long would it take for someone to build a three-decker?

Probably until a prestige-driven navy wanted the world's largest ship; weight and stability would demand a very broad, likely tumblehome hull.

Britain alone was expected to approach double-digit numbers of two-deck ironclads, so practical competition offered little reason for a third deck. Spain or another prestige-minded state might still build one to claim the world's largest ship. Britain could answer that single vessel with many conventional ironclads. The added upper weight would also demand greater beam and a carefully shaped, probably tumblehome hull to preserve stability.