Preheat oven to 375°.
Discard seeds and membranes from bell pepper. Carefully scoop out squash and zucchini pulp, leaving shells intact. Finely chop pulp; set aside. Place bell pepper halves, squash shells, and zucchini shells, cut sides up, in a 13 x 9-inch baking pan coated with cooking spray. Coat shells with cooking spray.
Place bread in a food processor; pulse 10 times or until crumbs measure 1 1/2 cups. Heat a large nonstick skillet over medium heat. Coat pan with cooking spray. Add chopped pulp and garlic; cook 4 minutes or until moisture evaporates, stirring frequently. Add tomato; cook 2 minutes or until tomato begins to soften, stirring frequently. Remove from heat. Stir in the breadcrumbs, 1/4 cup cheese, parsley, black pepper, and salt. Divide mixture evenly among prepared bell pepper halves, squash shells, and zucchini shells; sprinkle evenly with 1/4 cup cheese. Cover pan with foil, and bake at 375° for 40 minutes or until shells are tender.
Remove foil; broil 4 minutes or until cheese is lightly browned.