Thaw shrimp and scallops, if frozen. Halve any large scallops. Scrub clams under cold running water. Cover clams with salted water, using 3 tablespoons salt to 8 cups cold water. Soak for 15 minutes; drain and rinse. Discard water. Repeat twice. Set aside.
Cook spaghetti according to package directions; drain and keep warm. In a large skillet cook clams, yellow sweet pepper, onion, garlic, bouillon granules, basil, oregano, and pepper in the 1 cup water about 5 minutes or until vegetables are nearly tender and clams have opened. Remove clams; discard any unopened shells.
Stir together cornstarch and the 2 tablespoons cold water; stir into vegetable mixture. Cook and stir until thickened and bubbly. Stir in scallops and shrimp. Cook 3 to 4 minutes more or until scallops are opaque and shrimp turn pink. Stir in tomatoes and clams; heat through. Stir in parsley. Serve over spaghetti. Serve with Parmesan cheese. Makes 6 servings.