Preheat the oven to 375ºF. Grease a mini muffin pan and set aside.
In a large bowl, combine the melted butter, mashed bananas, shredded apple, shredded carrot, egg, and vanilla extract until well mixed.
Add in the cinnamon, baking powder, baking soda, and milk. Stir until well combined.
Add in the flour and oats, stirring until well combined.
Fill the mini muffin pan with batter and bake 18-20 minutes.
Let cool before serving.