N3 would likely evolve toward Nelson's layout in a no-treaty arms race

Would the N3 battleships have been built, and would their layout remain unchanged?

They had strong strategic justification, but their final layout would probably move toward the arrangement later seen in Nelson.

The finalized N3 would likely look more like Nelson than the early G3 arrangement, with the third turret moved ahead of the bridge and superstructure. That change reflects continuing design development rather than a frozen paper proposal. Without a treaty, Japanese, American, French, and Italian battleship programs would give British governments a straightforward political case for new construction. The case would be deterrence: maintaining a credible battle line in response to multiple potential rivals was cheaper than discovering in war that the fleet had fallen behind.