Soak mussels, clams, and shrimp in separate bowls of salted cold water for 1 hour. Drain. Clean and devein shrimp, reserving heads and shells.

Combine shrimp heads and shells, water, tomato, quartered onion, carrot, and celery in a large pot. Bring to a boil, reduce heat, cover, and simmer for 40 minutes to 1 hour until flavors combine. Season with salt and pepper.

Strain seafood stock through a fine mesh sieve and keep warm over low heat.

Heat 2 tablespoons olive oil in a large pot over medium heat. Add chopped onion and cook for 5 minutes. Stir in rice and cook until translucent, 2–3 minutes. Add 1 cup white wine and simmer until alcohol evaporates, about 3 minutes.

Gradually add stock, 1 ladle at a time, allowing the rice to absorb the liquid before adding more, stirring constantly. Continue for about 15 minutes until rice becomes creamy and almost tender.

In a separate saucepan, heat 1 tablespoon olive oil over medium heat. Add 1 clove garlic and cook until golden (1–2 minutes). Add mussels and 1/2 cup white wine, cover, and cook until mussels open (5 minutes). Remove mussels and strain cooking liquid through a sieve.

Repeat the above step for clams, using the same saucepan. Remove clams and strain their cooking liquid.

Heat remaining tablespoon olive oil in the same saucepan. Add garlic and shrimp, cooking until shrimp turns pink, about 2 minutes. Remove from heat.

Mix the reserved mussels' and clams' cooking water into the rice and cook for an additional 3–5 minutes. Fold in shrimp, mussels, and clams. Sprinkle parsley over the top.