Preheat oven for 350 degrees. Mash together two of the bananas in a large bowl. Set aside the other banana until later. Whisk egg, oil, coconut milk, lime zest and juice, and brown sugar into the mashed bananas. Set aside.
In a separate bowl, mix together flours, baking powder, baking soda, and salt. Slowly mix wet ingredients into dry ingredients, and mix until completely incorporated.
Line a 9x5 loaf pan with parchment paper. Pour batter into pan. Slice third banana length-wise. Top batter with sliced banana. Bake for 60-75 minutes or until a toothpick comes out clean from middle. Allow to cool prior to slicing.