The shipyard constraint behind a larger German capital-ship program

What would happen if Germany built a third Scharnhorst and Bismarck-class ship?

It would first require more industrial capacity or abandoning competing projects, then likely provoke a larger British response and alter treaty-era decisions.

Germany had a limited number of construction sites large enough for capital ships. Graf Zeppelin already competed for one of those scarce building positions, so an extra Scharnhorst or Bismarck required either new industrial investment or cancellation of another major project. Expansion would then create a strategic feedback loop rather than remain a purely German decision. Britain might order more King George V-class ships, reconsider treaty limits and gun calibre, or otherwise enlarge its own program in response.