By age 3, preschoolers have well-developed large motor skills. When learning to walk, most toddlers have difficulty moving backward. At this stage, children can now move forward and backward with ease, whether they are navigating stairs or walking backward.
Shelov, S. P. (Editor-in-Chief). (2004.) Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.