Classic Disney Shorts/Fridge

Fridge Horror

 * In the Classic Disney Shorts Donald's nephews were only staying with him until his brother-in-law got out of the hospital, after he was badly injured by a prank the triplets played on him. Since Huey, Dewey, and Louie never left, it implies their father died and the three are accidentally Self Made Orphans.
 * Actually, their mother left them there, but disappeared, along with the father, stranding them with Donald. And it was never spoken of again. The details depend on the author, though.
 * In Donald Duck comics, it is often mentioned that Donald is the sole guardian of Huey, Dewey, and Louie, which would imply that both their parents are dead.
 * Directly referenced in one of Don Rosa's comics, in which Uncle Scrooge mentions losing parents and one of the boys responds with "We know what that's like."
 * Speaking of the Don Rosa comics, if you consider the timeline canon and examine it carefully, you'll notice that Della had to have had the triplets when she was a teenager. That opens up all sorts of possibilities for why we haven't seen the father and why Della hasn't come back for her children.