By the end of Stave Three, what physical changes have occurred in the Ghost of Christmas Present?
At the end of Stave Three, we know it has been a long night and that the spirit has taken Scrooge to many places. Toward the end of this Scrooge realizes that his own outward appearance has not changed but that the spirit has aged. Specifically, he says,
"the Ghost grew older, clearly older. Scrooge had observed this change, but never spoke of it, until they left a children’s Twelfth Night party, when, looking at the Spirit as they stood together in an open place, he noticed that its hair was grey" (Stave Three).
This prompts Scrooge to ask the ghost if spirits' lives are so short - clearly indicating that he has aged so much before his eyes that he almost appears close to death (which is interesting given that the spirit is already dead). The spirit then confirms,
“My life upon this globe, is very brief,” replied the Ghost. “It ends to-night" (Stave Three).
So, while we do not know exactly how he looks, we know that he now has gray hair and he is looks SO old and so different from when he first began that his change in appearance is indicative of his own impending "end."
At the end of Stave 3, Scrooge noticed that the Ghost had aged noticeably from when he had first encountered him. He first noticed that his hair was gray, when at the beginning of the stave it had been dark brown. This is the only distinct, specific physical change that Dickens mentions, although it is intimated that the rest of his outward form has aged, as well. Dickens says that the Ghost had clearly grown older, so we readers can assume that he looks the opposite of the robust character from their introduction, and that the rest of his outward appearance has aged to match the graying hair.