Place racks in upper and lower thirds of oven; preheat to 425°. Toss carrots, cauliflower, and oil on a large rimmed baking sheet. Season with salt and toss again. Place sweet potatoes on a small foil-lined rimmed baking sheet. Roast veggies, tossing once, until browned and tender, 25–30 minutes for carrots and cauliflower and 35–40 minutes for potatoes.