Why Littorio survived three torpedo hits

Was the damage to Littorio surprising given her torpedo protection?

No. Three capable torpedoes hit her, but fortunate impact locations and harbour conditions kept the damage from becoming even worse.

British aerial torpedoes inflicted serious damage after substantial Royal Navy investment in the weapon. Littorio absorbed three hits, but their exact locations mattered as much as the nominal number. Her placement in shallow water and the distribution of damage kept the ship from suffering the worst possible outcome. Other Italian battleships were disabled for longer by a single well-placed hit. Torpedo protection cannot be judged from a hit count without examining where each warhead detonated.