Why the G3 and N3 programs were credible

Were the N3 battleships merely a bluff to force naval-treaty negotiations?

No; Britain possessed the technology, industrial capacity, and budget to build the planned ships.

Britain had the guns and shipyards needed to build the N3 battleships. The proposed construction could be distributed among several experienced yards, including government dockyards, and would sustain employment during demobilization. The G3 orders had already been allocated among private builders. Nelson and Rodney provide a comparison with capital ships Britain actually completed after the treaty. Britain's position after the First World War differed from its much more difficult circumstances after the Second World War.