In a large saucepan, saute the carrots, celery and onion in butter until tender. Stir in the turkey, water, potatoes, bouillon, salt and pepper. Bring to a boil. Reduce heat; cover and simmer for 10-12 minutes or until vegetables are tender.
Stir in 2 cups milk. Combine flour and remaining milk until smooth. Stir into soup. Bring to a boil; cook and stir for 2 minutes or until thickened.