1. Heat olive oil in a Dutch oven coated with cooking spray over medium-high heat. Add yellow onion, bell pepper, black pepper, and salt to pan; sauté 5 minutes or until tender. Add garlic; sauté 2 minutes. Add 1 3/4 cups water and next 10 ingredients (through chicken broth) to pan. Reduce heat to low, and simmer 12 minutes, stirring occasionally.
2. Arrange clams in a steamer. Cover and steam over boiling water 8 minutes or until shells open. Discard any unopened shells; set aside.
3. Add scallops and shrimp to tomato mixture; cook 7 minutes or just until done. Ladle 2 cups soup into each of 4 large shallow bowls; top each serving with 6 clams, 1 tablespoon cilantro, and 1 tablespoon green onions.