Preheat oven to 350 degrees. Spray 2 9-inch round cake pans with cooking spray.
Peel 2 bananas and mash them up in a bowl. Add cake mix, water, eggs and oil. Beat on low speed of mixer until well combined, about 1 minute. Increase speed to high and beat until mixture is smooth, about 3 minutes.
Chop remaining banana into 1/2-inch pieces. Using rubber spatula, fold bananas, chopped walnuts and chocolate chips into cake batter.
Divide batter evenly between pans.
Bake at 350 degrees for 30 minutes or until toothpick inserted in middle of cake comes out clean.
Cool in pans about 10 minutes. Remove from pans and cool completely.
Frost cake with your favourite chocolate frosting, putting icing between layers.