Seafood: Soak mussels, clams and prawns separately in cold salted water for about 1 hour. Clean and de-vein the prawns. Reserve the heads and the shells to make the stock.
Stock: Place prawn heads and shells in a large stock pot, then add 2 litres of cold water, onion, carrot, tomato and celery. Bring to the boil. Cover, reduce heat and simmer for 40 to 60 minutes. Season with salt and pepper. Strain the stock through a fine sieve and keep warm over low heat.
Risotto: Heat 2 tablespoons oil in a large pan and cook the onion until softened. Add the rice and toast for a few minutes, until the rice becomes translucent and has absorbed the oil. Turn up the heat, pour in 240ml of the wine and simmer until the alcohol has evaporated and the wine is mostly absorbed by the rice. Reduce heat to medium and begin to add the stock, a ladle at a time, stirring after each addition and adding the next one only when the first one is fully absorbed. The rice will take about 18 to 20 to cook fully.
Meanwhile, cook the seafood. In a large saucepan heat 1 tablespoon olive oil then add half of the garlic and cook gently for a few minutes until golden. Add mussels and half of the remaining white wine; cover and cook for 5 minutes, or until the mussels open. Turn off the heat, discard the garlic and any mussels that did not open. Remove the shells and keep the mussels warm. Pass the cooking liquid through a fine sieve and reserve for later.
Proceed exactly the same way with clams, using the remaining wine. removing the shells and reserving the cooking liquid for later.
Cook the prawns in a little oil with garlic for about 2 minutes, or until they turn pink. Set aside and discard the garlic.
Five minutes before the risotto is ready, pour in a little cooking liquid from the mussels and clams. Remove the pan from the heat when the rice is just slightly underdone, as it will continue cooking even off the heat.
Just before serving, add mussels, clams and prawns to the risotto. Turn off the heat, sprinkle with parsley and let stand a few minutes before serving.
It's best to cook mussels and clams separately since they vary in size and cooking times. Keep some shellfish in the shell to garnish the risotto.