Heat olive oil in a large skillet over medium heat. Add onions, bell peppers, and garlic; cook and stir until onions start to brown, 8 to 10 minutes.

Transfer mixture into a slow cooker. Stir in tomatoes, black beans, garbanzo beans, kidney beans, tomato paste, spinach, corn, zucchini, yellow squash, chili powder, cumin, oregano, parsley, salt, and pepper until thoroughly mixed.

Pour the tomato sauce and vegetable broth over the ingredients. Cook on Low for 4 to 5 hours. Check seasoning. If chili is too thick, add more tomato sauce and vegetable broth to desired thickness. Cook on Low for an additional 1 to 2 hours.