Versailles dismantled German capital-shipyard capacity

Did Blohm and Voss retain enough large slips in the mid-1930s to build three Scharnhorst-size ships at once?

No; the Treaty of Versailles had removed machinery and damaged or dismantled much of Germany's heavy naval construction capacity, which first had to be rebuilt.

A shipyard's earlier ability to hold several battlecruisers did not survive the postwar settlement intact. Allied removal of equipment and deliberate reduction of German naval infrastructure meant that nominal slip space in 1916 overstated the usable capacity available two decades later.