Add the carrots, celery and onion to the chicken soup base with the water. Cover and simmer 10 to 12 minutes, or until vegetables are tender. Blend the flour and 1 1/2 cups of the milk and add to vegetable mixture, stirring well. Add remaining milk and cook until thickened, about 15 minutes. Add cheese and stir just until melted. Season to taste with salt and pepper.