How does Marley and the three ghosts help to reform Ebenezer Scrooge?
At the beginning of the story, Scrooge is miserly and bitter, as evidenced by his wish that:
...every idiot who goes about with 'Merry Christmas' on his lips, should be boiled with his own pudding, and buried with a stake of holly through his heart.
Scrooge changes, however, as he encounters a series of apparitions. The first is Jacob Marley, his former business partner, who warns Scrooge that he must reform himself. Morley then announces that Scrooge will be visited by a series of apparitions, the famous Ghosts of Christmas Past, Present, and Yet to Come. The first spirit takes him back to when he was a young man, and and apprentice of Old Fezziwig, a kindly old man that Scrooge remembers fondly. He also sees his former fiancee Belle, who left him due to his miserly ways.
The second spirit shows him the house of his employee Bob Crachit. Scrooge is particularly moved by the plight of Tiny Tim, Crachit's sickly old son. He is also exposed to the poverty and squalor in which many people are forced to live on Christmas. The third spirit shows Scrooge a future in which he has just died. People are not only not sad to see him dead, but are actually relieved, especially his debtors. He will die unmourned and alone.
Scrooge is profoundly moved by these experiences, and arises on Christmas Day a changed man. He goes out of his way to be generous to all, and even spends money to pay for medical care for Tiny Tim.