Could a third Lord Nelson-class battleship have been completed while HMS Dreadnought was building?
Short answerYes. The two historical ships were themselves completed only after Dreadnought had entered service.
NotesHMS Lord Nelson and Agamemnon were completed after HMS Dreadnought had already entered service. The arrival of the new battleship did not stop completion of those two pre-dreadnoughts. A hypothetical third Lord Nelson-class ship could likewise have been finished while Dreadnought was being built.