Position racks in the upper and lower thirds of the oven and preheat to 450 degrees . Combine the bell peppers and onions on a large rimmed baking sheet. Sprinkle the garlic, 1 tsp. salt and 1/2 tsp. pepper on top. Toss with 3 tbsp. EVOO and spread evenly in the pan. Place in the lower third of the oven and cook for 10 minutes.
Meanwhile, place the butternut squash on another large rimmed baking sheet. Drizzle with the remaining 2 tbsp. EVOO and sprinkle with 1/2 tsp. salt and 1/4 tsp. pepper; toss and spread evenly in the pan. Once the peppers and onions have cooked for 10 minutes, add the squash to the upper third of the oven and roast, along with the peppers and onions, until fork-tender, 15 minutes. Let the vegetables cool completely on their pans.