In a small saucepan, combine the cranberries with the sugar and water and bring to a boil. Simmer over moderate heat, stirring occasionally, until the cranberries soften, about 5 minutes. Mash the cranberries with the back of a spoon to make a thick sauce. Transfer the sauce to a large bowl and refrigerate, stirring often, until cool, about 15 minutes. Stir 3 cups of the buttermilk into the cooled cranberry sauce.