During their third year, children are in constant motion–running, kicking, climbing, and jumping. They accomplish several major motor milestones during this time period, including alternating feet while walking up and down stairs, kicking and directing the motion of a ball, and standing on one leg (with support). At this age, their running also becomes smoother and more coordinated.
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.