1. Cook potatoes and carrots in separate medium saucepans of boiling salted water until tender, about 25 minutes. Drain potatoes and carrots. Return each vegetable to its own pan. Mash potatoes; stir over low heat to dry slightly, about 2 minutes. Mix in whipping cream and 3 tablespoons butter. Mash carrots coarsely in pan. Mix into potatoes. Season mixture to taste with salt and pepper. (Mashed potatoes can be prepared 2 hours ahead. Let stand at room temperature. Rewarm over low heat before serving.)
2. Transfer mashed potatoes to large bowl and serve hot.