Step 1
Soak mussels clams and shrimp in separate bowls of salted cold water for 1 hour. Drain. Clean and devein shrimp reserving heads and shells.
Step 2
Combine shrimp heads and shells 2 quarts water tomato quartered onion carrot and celery in a large pot; bring to a boil. Reduce heat cover and simmer until flavors combine 40 minutes to 1 hour. Season with salt and pepper.
Step 3
Strain stock through a fine-mesh sieve discarding solids. Keep warm over low heat.
Step 4
Heat 2 tablespoons olive oil in a large pot over medium heat. Add chopped onion; cook and stir until softened about 5 minutes. Stir in rice until translucent and coated with oil 2 to 3 minutes. Increase heat to medium-high; pour in 1 cup white wine. Simmer until alcohol has evaporated about 3 minutes.
Step 5
Reduce heat to medium and stir in 1 ladle of stock until rice has absorbed the liquid. Repeat with remaining stock stirring after each addition until rice is creamy and almost tender about 15 minutes.
Step 6
Heat 1 tablespoon olive oil in a large saucepan over medium heat. Add 1 clove garlic and cook until golden 1 to 2 minutes. Add mussels and 1/2 cup white wine; cover and cook until mussels open about 5 minutes. Remove from heat; discard garlic and any mussels that did not open. Remove shells and keep mussels warm. Strain cooking liquid through a fine-mesh sieve into a bowl.
Step 7
Heat 1 tablespoon olive oil in the same saucepan over medium heat. Add 1 clove garlic and cook until golden 1 to 2 minutes. Add clams and 1/2 cup white wine; cover and cook until opened about 5 minutes. Remove from heat; discard garlic and any clams that did not open. Remove shells and keep clams warm. Strain cooking liquid through a fine-mesh sieve into a bowl.
Step 8
Heat remaining olive oil in the same saucepan. Add 1 clove garlic and shrimp; cook and stir until shrimp turn pink about 2 minutes. Remove from heat and discard garlic.
Step 9
Pour some of the mussel and clam cooking water over the rice; cook for 3 to 5 minutes more. Remove rice from heat. Fold in mussels clams and shrimp. Sprinkle parsley on top. Let stand before serving about 5 minutes.