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 OLD BAY Seasoning. Cook for 1 minute stirring constantly.
Add chicken broth (undiluted) and milk to the seasoned vegetables. Cook over medium heat for 25 minutes.
Add reserved haddock, shrimp and crab meat. Cook for an additional 10 minutes. Stir in shredded cheese. Continue to stir and cook until cheese melts. Serve immediately.
