Preheat the oven to 475°. Quarter the eggplant, zucchini and yellow squash lengthwise, then trim off the central portion with the seeds. Cut the vegetables into 1/2-inch dice. In a large bowl, toss the vegetables with 1/2 cup of the olive oil and the garlic. Season with salt and pepper. Spread the vegetables on 2 rimmed baking sheets and roast for 25 to 30 minutes, stirring twice, until tender.