Add the black beans, kidney beans, pinto beans, diced onion, and diced tomatoes to the slow cooker.
Stir in the chili powder, cumin, garlic powder, and salt until everything is evenly mixed.
Cover and cook on low for 6 hours so the flavors blend and the chili thickens.
About 30 minutes before the chili finishes, heat the oven to 400 degrees and grease an 8-inch baking pan.
In a mixing bowl, whisk together the cornmeal, flour, baking powder, and salt.
Add the milk, egg, and vegetable oil to the dry ingredients and stir until just combined.
Pour the batter into the prepared pan and bake for 20 minutes until the top turns golden.
Let the cornbread cool for a few minutes, then cut it into squares and serve alongside the chili.