Peel the potatoes and turnip, and cut them into 2-inch pieces.
Place potatoes and turnips in a saucepan with the garlic, salt and water, and bring to a boil. Cover, reduce the heat and boil gently for 20 minutes, or until the vegetables are tender. (About 1/2 cup of liquid should remain. If necessary, add a splash of water or stock; or, if too much liquid remains in the pan, remove the lid and quickly boil to reduce.)