Whisk together balsamic vinegar, garlic, 2 tablespoons oil, and salt and pepper to taste.
Preheat broiler. Toss zucchini, yellow squash, and bell peppers with half of dressing in a large bowl, then marinate 5 minutes.
Broil half of dressed vegetables on oiled rack of a broiler pan 5 to 7 inches from heat, turning over once, until golden brown and tender, about 16 minutes total. Transfer to a cutting board, then broil remaining dressed vegetables in same manner. When cool, cut into 1-inch pieces.
While vegetables are broiling, sauté onion in remaining tablespoon oil in a 4-quart heavy saucepan over moderately high heat, stirring, until softened, then add couscous and sauté, stirring, 2 minutes. Add broth and simmer, covered, until couscous is just tender, 8 to 10 minutes. Stir in broiled vegetables, basil, remaining dressing, and salt and pepper to taste. Serve at room temperature.