Preheat oven to 425 degrees F. Combine the sweet potatoes, carrots and onions in a large bowl. Drizzle with the olive oil, adding more if necessary to lightly coat, sprinkle with the seasonings and salt and pepper. Toss to coat. Spread into an even layer on a lightly greased baking sheet. Roast for 25-30 minutes, turning halfway through. Vegetables should be soft and lightly browned when done. Season with additional salt and pepper if desired. Serve hot.