Preheat a grill to medium high. Meanwhile, bring a large pot of salted water to a boil. Add the couscous and cook until just tender, about 8 minutes. Drain and transfer to a large bowl; toss with 1 tablespoon olive oil and season with salt and pepper.
Meanwhile, soak the red onion in a small bowl of hot water, 10 minutes. Toss the bell peppers and zucchini in a large bowl with the remaining 2 tablespoons olive oil, the cumin, 1/2 teaspoon salt and a few grinds of pepper. Grill the vegetables, turning once, until well marked and tender, 8 to 10 minutes. Transfer to a plate and let cool to room temperature. Cut the peppers into 1-inch pieces and slice the zucchini into half-moons.
Drain the onion and add to the couscous along with the bell peppers, zucchini and chickpeas. Add the dressing and toss; season with salt and pepper.
Green Goddess Dressing
Combine 1/2 cup each mayonnaise and sour cream, 1/3 cup each chopped parsley and chives, and 1/4 cup chopped tarragon in a blender. Add 2 anchovy fillets, 1 minced garlic clove, 1 tablespoon capers, the juice of 1 lemon, 1/2 teaspoon kosher salt and a few grinds of pepper; puree until smooth. (Makes 1 1/2 cups.)