In a large bowl blend together softened cream cheese, bleu cheese, and cheddar cheese. Then add buffalo sauce, dried onion, minced celery, and parsley. Mix well again and lastly stir in the chicken.
Clean mushrooms and remove stems. On a large baking sheet add a metal cookie cooling rack to the baking sheet. Then begin to fill each mushroom cap with the buffalo chicken mixture. Place each mushroom on the rack.
Bake for 20 minutes at 450 degrees. Serve immediately.