The first ghost symbolizes the truth of Scrooge's abusive "past;" as a child, Scrooge was neglected during the most beautiful holiday of the year, Christmas; we see two Christmases where Scrooge is left alone at a boarding school while all the other boys are going home for Christmas. It's no wonder that Scrooge dislikes Christmas so much; as a child he had no psychological tools to deal with this problem; these lonely Christmases scarred Scrooge deeply. He must have felt that no one loved him.
The second ghost symbolizes what Scrooge is suppressing, the joy of all that Christmas represents: family, giving, kindness, forgiveness, God, and love. Scrooge has buried these abstract qualities because of his Childhood experiences.
The third ghost represents what will happen to Scrooge because of his mental attitude. Because Scrooge is so scarred mentally by his past Christmases, he has determined that it is better to shut off all loving emotions associated with Christmas and people, and in doing so he has condemned himself to living and dying alone and also affecting the lives of others in a most negative way, particularly the Cratchits.
Scrooge had a terrible childhood; he suffered the neglect of a family, and he suffered this neglect during Christmas which led him to loath family and Christmas. His loathing of both is demonstrated in the fact that he doesn't have a family, he refuses his nephew's invitiations to Christmas dinner and of his philosophy of Christmas being a "humbug."
The Ghosts in "A Christmas Carol" serve to reflect the things that Scrooge needs to help him to change.
The first ghost is the Ghost of Christmas past. He comes after the ghost of his partner Marley. The ghost has chains on him. The chains are attached to cash boxes. He is symbolic of Scrooges greed. He is chained to what he found valuable in life.
The second ghost is dressed in a green robe and rests upon a throne. He serves to demonstrate to Scrooge what he is missing in the present and what others are presently experiencing. He takes Scrooge to the Cratchits so Scrooge can see what love and family are despite the hardships that the family has to endure. His role is very significant. I have read before that the throne serves to show how Scrooge has put himself on a pedestal. Yet, it is very lonely on the throne alone. This ghost speaks and echoes back some of Scrooges own comments such as:
"Are there no prisons? Are there no workhouses?"
The third ghost is the ghost of Christmas future. He is a vague creepy phantom. He is there to demonstrate the foreboding events that will come to pass. His presence is basically there to show that Scrooge's choices are leading to a horrible end. He is like the grim reaper. He does not talk but points. He gives no hints as to whether the future can be changed.