Explain the main differences and similarities of the three spirits in A Christmas Carol.
You have asked quite a big question here, and the best way to answer it is to compare and contrast the way that the text introduces and describes the three ghosts. You are right in being aware of the general symbolism of each ghost and the time that they represent, but it is important to realize how each ghost operates to taunt Scrooge with the kind of man that he is now and with regret and remorse over his past, present and future actions.
The Ghost of Christmas Past is described as being:
...like a child: yet not so like a child as like an old man, viewed through some supernatural medium, which gave him the appearance of having receded from the view, and being diminished to a child's proportions.
It is perhaps fitting then that the ghost that examines the past and the actions and choices that have led Scrooge to become the man that he is today appears in such a way that its very appearance depicts looking back on our past lives and decisions.
The Ghost of Christmas Present is described in such a fashion as to emphasize the season of plenty and the festival that he represents:
...there sat a jolly Giant, glorious to see; who bore a glowing torch, in shape not unlike Plenty's horn, and held it up, high up, to shed its light on Scrooge, as he came peeping round the door.
Perhaps this description is fitting for the Ghost that shows so many people enjoying the season and all of the luxuries and warmth and festivities that come along with it.
Lastly, the Ghost of Christmas Yet To Come is described in such a way as to emphasize its darkness and associations with death:
It was shrouded in a deep black garment, which concealed its head, its face, its form, and left nothing of it visible save one outstretched hand. But for this it would have been difficult to detach its figure from the night, and separate it from the darkness by which it was surrounded.
Again, the point of this Ghost is to show how Scrooge is remembered after his death, and to shock him with how little respect and regard he had even from those he considered his "friends" and "business associates." Thus the darkness and the "solemn dread" that this Ghost inspires in Scrooge is perhaps fitting.
Key to understanding these Ghosts is considering the way they are described and the scenes that they show Scrooge. I hope I have given brief indications of how each of the Ghosts operate. Good luck!