Spread pecans, almonds, and walnuts evenly in a 15- x 10- x 1-inch jellyroll pan. Bake at 250° for 20 minutes or until lightly toasted. Remove from oven; set aside.
Beat egg whites (at room temperature) and salt in a large mixing bowl until soft peaks form. Gradually add sugar, 1 tablespoon at a time, beating until stiff peaks form. Fold in toasted pecans, almonds, and walnuts.
Pour melted butter into jellyroll pan. Spread nut mixture evenly over butter. Bake at 300° for 50 minutes, stirring every 15 minutes. Let cool completely. Store in airtight containers.