In a large mixing bowl, mash the 3 ripe bananas with a fork until smooth but with some lumps for texture. Add the melted butter, granulated sugar, brown sugar, egg, and vanilla extract. Whisk together until well combined and glossy.

Sprinkle the flour, baking soda, salt, and cinnamon over the wet mixture. Gently fold everything together with a spatula until just combined, being careful not to overmix. If using add-ins, fold them in now.

Pour the batter into the prepared slow cooker and smooth the top. Cover and cook on high for 3 hours or on low for 5-6 hours, until a toothpick inserted into the center comes out clean or with a few moist crumbs.

Turn off the crockpot and let the bread cool in the slow cooker for 15-20 minutes. Carefully remove using the parchment paper and transfer to a wire rack to cool completely before slicing.