The visitations do happen over the course of three days as seen by that Scrooge goes to bed at past two in the morning, the first two visitations have Scrooge awakening shortly after midnight, and waiting to hear the clock strike one before the ghost appears. As per Marley's statement, the third ghost, the the Ghost of Christmas Yet To Come appears right after midnight. Scrooge does note the first time passage and is astonished. At the second, he is so bemused from his experience that he does not know what to expect. And, of course, the third ghost appears in the wake of the second.
First Ghost
To his great astonishment the heavy bell went on from six to seven, and from seven to eight, and regularly up to twelve; then stopped. Twelve! It was past two when he went to bed. The clock was wrong. An icicle must have got into the works. Twelve!
....
“Why, it isn’t possible,” said Scrooge, “that I can have slept through a whole day and far into another night. It isn’t possible that anything has happened to the sun, and this is twelve at noon!”
....
“The hour itself,” said Scrooge, triumphantly, “and nothing else!”
He spoke before the hour bell sounded, which it now did with a deep, dull, hollow, melancholy One. Light flashed up in the room upon the instant, and the curtains of his bed were drawn.
....
The Spirit dropped beneath it, so that the extinguisher covered its whole form; but though Scrooge pressed it down with all his force, he could not hide the light: which streamed from under it, in an unbroken flood upon the ground.
He was conscious of being exhausted, and overcome by an irresistible drowsiness; and, further, of being in his own bedroom. He gave the cap a parting squeeze, in which his hand relaxed; and had barely time to reel to bed, before he sank into a heavy sleep.
Second Ghost
Awaking in the middle of a prodigiously tough snore, and sitting up in bed to get his thoughts together, Scrooge had no occasion to be told that the bell was again upon the stroke of One....
Gentlemen of the free-and-easy sort, who plume themselves on being acquainted with a move or two, and being usually equal to the time-of-day, express the wide range of their capacity for adventure by observing that they are good for anything from pitch-and-toss to manslaughter; between which opposite extremes, no doubt, there lies a tolerably wide and comprehensive range of subjects. Without venturing for Scrooge quite as hardily as this, I don’t mind calling on you to believe that he was ready for a good broad field of strange appearances, and that nothing between a baby and rhinoceros would have astonished him very much.
Now, being prepared for almost anything, he was not by any means prepared for nothing; and, consequently, when the Bell struck One, and no shape appeared, he was taken with a violent fit of trembling....
Third Ghost
The bell struck twelve.
Scrooge looked about him for the Ghost, and saw it not. As the last stroke ceased to vibrate, he remembered the prediction of old Jacob Marley, and lifting up his eyes, beheld a solemn Phantom, draped and hooded, coming, like a mist along the ground, towards him.
Given that the second ghost, ostensibly showing him the present, brings him to Christmas Day, it seems that the ghosts are not moored to time, and therefore can rewind the day as necessary to bring him all three visits, and have it still be Christmas Day upon his waking for the final time. As far as I know, Dickens never commented on the timing of the visits, but I have seen commentaries linking it to the three days before Jesus rose from the dead (along with more tying it to being Christian allegory), or to Scrooge being a man who lived only in the present, having rejected his past and gathering money not for any future purpose, but to have it now (and yet, doing nothing with it, living in virtual privation), and therefore he was already detached from time.
