Melt the 3 tablespoons of butter or margarine. In a medium mixing bowl, combine walnuts, butter or margarine, and sugar. Press this nut mixture firmly into bottom and sides of a 9 inch pie plate. Bake at 325 degrees F (165 degrees C) for about 10 minutes, or until the edge is golden brown. Cool.
Chop the candy bars up into small pieces. In a medium saucepan, combine the chopped bars of chocolate, marshmallows, and milk; heat and stir over medium heat till melted. Remove from heat; cool to room temperature.
Chill a large mixing bowl and beaters. In the chilled bowl, beat whipping cream and vanilla with electric mixer on medium speed till soft peaks form (tips curl). Fold whipped cream into cooled chocolate mixture.
Spoon chocolate mixture into the pie crust. Freeze about 5 hours or until firm. Remove pie from the freezer about 10 minutes before serving. If desired, garnish with additional whipped cream and chopped chocolate bars.