1Combine butter, milk and sugar in large heavy saucepan. Stir over medium heat until sugar is dissolved; bring to a boil. Cover and boil for 5 minutes, turn of heat, add marshmallows, stir until dissolved.
2Add the 3 kinds of chocolate, one at a time. Stirring after each addition, until completely melted. Add vanilla and nuts. Stir to blend.
Pour into lightly greased 10x13 inch pan; let stand until cool and firm before cutting.