Heat olive oil in a large skillet over medium heat, and cook the green, red, and yellow bell peppers, onions, and garlic until the onions start to brown, 8 to 10 minutes. Place the mixture into a slow cooker. Stir in spinach, corn, zucchini, yellow squash, chili powder, cumin, oregano, parsley, salt, black pepper, tomatoes, black beans, garbanzo beans, kidney beans, and tomato paste until thoroughly mixed. Pour the tomato sauce and vegetable broth over the ingredients.
Set the cooker on Low, and cook until all vegetables are tender, 4 to 5 hours. Check seasoning; if chili is too thick, add more tomato sauce and vegetable broth to desired thickness. Cook an additional 1 to 2 hours to blend the flavors.
