In a Dutch oven, combine the potatoes, parsnips, celery roots, broth and 2 teaspoons salt. Add water to cover vegetables. Bring to a boil. Reduce heat; cook, covered, for 15-20 minutes or until vegetables are tender. Using a slotted spoon, transfer vegetables to a large bowl; reserve cooking liquid. Mash vegetables, gradually adding the milk, garlic powder, pepper, remaining salt and enough reserved cooking liquid to reach desired consistency.