my most useful source on the design evolution of the Scharnhorst-class is the old "Marine Arsenal Band 24 - Die Schlachtschiffe der Scharnhorst Klasse" by Siegfried Breyer.
It doesn't go into too much detail, but includes a few small line-drawings and most importantly some original design plans for the later proposals (the quality is mediocre, though).
Koop & Schmolkes "Battleships of the Scharnhorst Class" might also be of interest, eventhough the part about the development history is rather brief.
A quick google search also wound up a plan of the initial design 3b:
https://frm-wows-us.wgcdn.co/wows_forum ... 140262.jpg
Hope that helps!