Grease the inside of your slow cooker with butter or non-stick spray.

In a large mixing bowl, mash the 3 ripe bananas until mostly smooth with a few lumps remaining.

Pour the melted butter over the mashed bananas. Add the granulated sugar, brown sugar, egg, and vanilla extract. Stir everything together with a fork or wooden spoon until just combined.

Sprinkle the flour, baking soda, salt, and cinnamon over the wet mixture.

Gently fold the dry ingredients into the wet mixture using a spatula or wooden spoon until no streaks of flour remain. Do not overmix.

Pour the batter into the prepared slow cooker. Cover and cook on low for 2 to 2.5 hours, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Turn off the slow cooker and remove the lid. Let the bread cool in the slow cooker for at least 30 minutes before carefully removing it. Slice and serve warm.