In a large saucepan cover rutabagas with salted water. Bring to a boil and simmer, covered, for 30 minutes.
Add carrots and potatoes, and cook another 15 – 20 minutes until the vegetables are tender.
Reserve 1 1/2 cup cooking liquid, and drain vegetables well in a large sieve.
Return vegetables to pot. Add butter pieces and mash with a potato masher to desired consistency (I like to make it somewhat chunky rather than baby-food-like).
Add reserved cooking liquid, mix well, season with salt and pepper.
It can be made three days ahead. Place in baking dish, cover and refrigerate.
Place in a preheated oven for 30 minutes or until hot before serving.