This is the original release three-sheet. It has dimensions of 41" x 81", which is equivalent to three one-sheets (go figure). All three-sheets of this period were printed in two pieces, and occasionally you'll get a pair that don't line up that well, but this one has great registration.
What's interesting about this poster is the art. It is the exact same image as on the one-sheet, but more of it. If you notice on the one-sheet, you cannot see the Cheshire Cat above the Mad Hatter, you only get a fragment of his tail.