Combine sugar, water and vinegar in a heavy saucepan. Cook over medium heat, stirring frequently until mixture comes to a boil and sugar disolves. Continue cooking, stirrin frequently until mixture reaches soft ball stage (240~). Beat egg whites until foamy. While beating at medium speed, slowly pour hot syrup in a thin stream over egg whites. Turn mixer to high speed and continue beating until stiff peaks form and frosting is thick enough to spread. Fold in marshmallows. Enough to cover a 3-layer cake.