By age 3, most children speak clearly enough that strangers can understand most of what they say. Despite this, children may continue to mispronounce many of the speech sounds they use. They may transpose w and r or replace d with th. The sounds made by the letters b, p, m, w, and h usually surface midway through the third year, and it may take a few months for children to use them perfectly.
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.