How early could the Royal Navy have adopted triple main-battery turrets?
Short answerIt was technically possible around the Lord Nelson era, but production efficiency, layout preferences, and a desire for more hulls favored twins.
NotesThe Royal Navy already had twin mountings in production and designed ships around numerous turrets, so changing systems during the dreadnought race risked delay. A triple mounting might have appeared with the Lord Nelson class and encouraged superfiring layouts, but it was too wide for wing positions. Britain also valued distributing a fixed number of guns across more ships, since its finances and shipyards did not impose a hard hull limit.