Preheat oven to 350℉. Lightly grease 12 muffins cups, or line with muffin liners.
Mix together1 ½ cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon baking powder, ½ teaspoon cinnamon, and ⅛ teaspoon nutmeg. In another bowl, mix together mashed 3 ripe bananas, ½ cup granulated sugar, ¼ cup light brown sugar, 1 large egg, and melted ¼ cup butter.
Add banana mixture into the flour mixture and stir until just moistened. Scoop 3 tablespoons of batter in each muffin cup (about ¾ full) and lightly tap pan on counter to even out the tops.
Bake at 350℉ for 16-20 minutes or until toothpick comes out clean.