Preheat the oven to 375℉. Grease a mini muffin pan with cooking spray and set aside.
In a large bowl, mix together the bananas, apple, carrot, milk, oil, egg and vanilla until combined.
In a medium bowl, mix together the flour, oats, cinnamon, baking powder and baking soda.
Slowly mix the dry ingredients into the wet ingredients until just mixed together.
Divide the batter into the prepared muffin pan and bake in the preheated oven for 16-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow to cool in the pan for around 5 minutes before transferring to wire racks to cool completely.