Preheat oven to 375°F. 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 1/2 the parmigiano while tossing. Place in a buttered casserole and sprinkle the remaining parmigiano on top. Sprinkle the nutmeg over everything and bake for 15-20 minutes. When the top of the pasta has turned golden brown, it is done.