Preheat oven to 350 degrees. Line 18 standard sized muffin cups with paper liners.

In a medium bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.

In a large mixing bowl, mash 2 of the bananas well with a fork. Add the melted butter, egg, milk, and vanilla, and whisk well. Add the banana mixture to the flour mixture and stir until all the flour is blended into the batter. Dice the remaining banana and fold into the batter along with the nuts. Divide the batter evenly among the prepared muffin cups.

Bake the muffins for 30 – 35 minutes, or until golden brown and a toothpick inserted into the center comes out clean. Let cool for 5 minutes in the pan.