In a saucepan, combine soup and cream cheese; cook and stir over medium heat until smooth. Remove from heat and stir in mayonnaise.
Meanwhile in a small bowl, sprinkle gelatin over hot water. Let stand 5 minutes or until dissolved. Add gelatin to hot soup mixture; stir until dissolved. Add onions, celery and crab; stir to combine. Transfer to a serving bowl. Cover and chill for 2-3 hours or until set. Serve with crackers.