Warm oil in a large skillet over medium-high heat. Add carrots, celery and onion and cook, stirring often, until softened, 3 to 5 minutes. Stir in curry powder and sauté until blended and fragrant, about 1 minute. Add rice and stir to coat. Stir in tomatoes and broth. Bring to a simmer, then pour into slow cooker. Stir in lentils and season with salt and pepper. Cover and cook on low for 4 hours. Add chickpeas, cover and cook until lentils and rice are tender, about 1 hour longer. (If stew looks dry when you add chickpeas, add more broth.) Season with salt and pepper and sprinkle with parsley, if desired, before serving.