Heat olive oil in the cazuela or skillet over medium-high heat. Add onions, garlic, and red bell pepper, sauté until softened.
Stir in tomato paste and saffron threads, allowing the flavors to meld for a few minutes.
Add Arborio rice and cook for another 2-3 minutes, stirring to coat the rice with the saffron-tomato mixture.
Pour in the white wine and simmer until it's mostly evaporated.
Gradually add chicken or vegetable broth, one cup at a time, stirring frequently. Let the liquid be absorbed before adding more.
When the rice is almost tender, nestle the seafood (shrimp, mussels, and clams) into the rice.
Cover the cazuela and simmer until the seafood is cooked and the rice is creamy and slightly crispy on the bottom. Ensure the internal temperature of the shrimp reaches 145°F (63°C).
Season with salt and pepper to taste and garnish with fresh parsley. Serve hot with lemon wedges.