Why Bismarck's three shafts traded away survivability

Why was Bismarck designed with three propeller shafts rather than four?

Probably to save weight and hull space, although four shafts would have been better for survivability.

The three-shaft choice is interpreted as a way to manage weight and machinery space within the hull. That may help explain the German design decision without making it optimal. Four shafts would improve survivability by distributing damage across more propulsion paths. Bismarck's arrangement therefore illustrates how a local machinery compromise can create wider combat consequences.