Heat the olive oil in a large oven-safe skillet over medium heat. Add the onion, garlic, and bell pepper. Cook until the vegetables soften.
Stir in the black beans, kidney beans, pinto beans, diced tomatoes, chili powder, cumin, and salt. Simmer the mixture for 10 minutes to thicken slightly.
In a mixing bowl, combine the cornmeal, flour, and baking powder. Add the milk, egg, and vegetable oil. Stir until a thick batter forms.
Pour the batter evenly over the chili in the skillet. Transfer the skillet to a 400°F oven and bake for 20 minutes until the topping is golden and a toothpick inserted in the center comes out clean.
Remove from the oven and let it rest for 5 minutes before serving.