Preheat oven to 350 degrees F. In a large bowl, cream the margarine and then gradually add the sugar substitute, beating with an electric mixture until light and fluffy. Add the eggs, and continue to beat until the mixture is thick and lemon colored.
In a separate bowl, combine the flour and baking powder . Add it to the creamed mixture and mix well. Fold in the bananas and apple.
Spoon the batter into an 8 1/2x4 1/2x3-inch loaf pan coated with cooking spay. Bake for about 65 to 70 minutes, or until a wooden toothpick inserted into the center comes out clean. Remove from the oven and allow the loaf to cool in the pan for 10 minutes. Afterwards, remove the loaf from the pan and allow to cool completely on a wire rack.