In a large bowl, combine the flour, baking soda, and salt; set aside. Mash 2 of the bananas with a fork in a small bowl so they still have a bit of texture. With an electric mixer fitted with a wire whisk, whip the remaining bananas and sugar together, for about 3 minutes. Add the melted butter, eggs, and vanilla and beat well. Mix the dry ingredients until incorporated. Fold in the nuts and the mashed bananas with a rubber spatula. Spoon the batter into the muffin tins and fill them about 2/3 the way.