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."