Preheat oven to 375 degrees. In lots of boiling water, add the salt and rigatoni. Cook until super al dante as they are going into the oven. Drain and rinse in cold water. In a large bowl, mix the butter into the pasta until it is well coated. Add the three cheeses and the cream. Toss well and add half the parmigiano while tossing. Place in a buttered casserole and sprinkle the remaining parmigiano on top. Sprinkle the nutmeg over everything and bake for 15 to 20 minutes. When the top of the pasta has turned golden brown, it is done.