Quarter zucchini lengthwise; cut crosswise into 1/2-inch-thick pieces. Remove ribs and seeds from peppers; cut into 3/4-inch squares.
Heat olive oil in a large skillet over medium-high heat. Add zucchini, peppers, tomatoes, and garlic; season with salt and pepper. Cook, tossing frequently, until vegetables are crisp-tender, 6 to 8 minutes.