Today, in 1937, Huey Dewey and Louie made their first appearance in a newspaper comic strip. They didn't make their first theatrical appearance until April 15, 1938.
Donald's nephews belong to his sister, Della Thelma Duck, or "Dumbella" as she has been called. The three boys were originally only to spend a day with their uncle while their father was at the hospital. Why was he in the hospital? He was recuperating from a practical joke the boys played on him dealing with firecrackers...
Somewhere along the way their parents disappeared into the bowels of Disney animation and the boys became a permanent part of Donald's life.
What you may not know is that there was a short-lived fourth nephew "Phooey Duck" who made a few appearances. He was explained away as a "freak incident of nature" and never heard from again!
Source - This article is based on information available from wikipedia.