This is not a South Beach recipe, but I suppose it could be converted into a phase 3 recipe by changing the flour and sugar; I have never tried such a recipe, though.
1/2 cup margarine (or butter)
1 cup white sugar
2 cups all-purpose flour
1 tsp. baking soda
3 overripe bananas
1/4 cup chopped walnuts
Mix all ingredients - add bananas last.
Baked in a greased loaf pan at 325 degrees for 1 hour, 15 minutes.