Heat mushroom soup and cream cheese in a sauce pan, stirring until smooth. Remove from heat. Add mayonnaise. Dissolve gelatin in 3 Tbsps. very hot water and stir immediately into mixture. Mix very well. Stir in celery, onion and crab (crab should be diced fairly fine).Pour into a mold or 2-lb. container. Refrigerate over night. To unmold, dip in hot water just below rim for a few seconds. Place plate or serving tray over top of container and invert quickly.