Thaw clams, if frozen. Scrub clam shells under cold running water using a stiff brush. Combine 8 cups water and 3 tablespoons salt. Add clams; soak 15 minutes. Drain and rinse. Discard water. Repeat the soaking, draining, and rinsing steps two more times.
Partially thaw fish and shrimp, if frozen. Remove and discard fish skin, if present. Cut fish into 1-1/2-inch pieces; set aside. Cover and refrigerate fish pieces and shrimp until needed.
In a large saucepan cook mushrooms, green or red sweet pepper, onion, and garlic in hot oil until tender but not brown. Stir in undrained tomatoes, wine, water, parsley, tomato paste, lemon juice, basil, oregano, sugar, salt, and crushed red pepper. Bring to boiling; reduce heat. Simmer, covered, 20 minutes.
Add clams, fish pieces, and shrimp. Simmer, covered, for 5 to 10 minutes or until clams open, fish flakes easily, and shrimp are opaque. Discard any unopened clams. Makes 4 servings.