HEAT oil in large skillet on medium-high heat. Add eggplant, zucchini and yellow squash; cook 10 min., stirring frequently. Stir in garlic; cook on medium heat 5 to 8 min. or until vegetables are tender. Remove vegetables from skillet; set aside. Mix ricotta and Parmesan.
SPREAD 1 cup spaghetti sauce onto bottom of skillet; top with small spoonfuls of ricotta mixture, cooked vegetables, remaining spaghetti sauce and mozzarella. Cover.
COOK on medium-low heat 20 min. or until heated through. Remove from heat; let stand 5 min. before serving.