Thaw frozen fish enough to cut it into cubes. Reserve.
In a large saucepan, melt the butter or margarine. Add onions, celery, carrots, and potatoes and cook until onions are transparent. Carefully blend in the flour, salt, and seasoning. Cook for 1 minute, stirring constantly.
Add chicken broth (undiluted) to the milk and seasoned vegetables and cook over medium heat for 25 minutes. Add reserved haddock, shrimp, and chopped crab meat and cook for an additional 10 minutes.
Stir in shredded cheese. Continue to stir and cook until cheese melts. Serve immediately.