How did Donald Duck get custody of his three nephews?

Answer 1

Donald got a letter from Della, (in the comic strip refered to as his cousin but in the animation version refered to as his sister, eventually officialized as his sister also in the comics world.) In the letter she said she was sending his three "angel nephews" to stay with him while the boys' father is in hospital because a giant firework rocket exploded under the father's arm chair. She clearly indicated it was the boys' doing. Also in the comics she soon calls Donald and tells she has to leave town for a few days and askes Donald to care for the boys while she's gone. The parents were never shown nor refered to again, as in it has just been commonly visioned by Disney's storywriters that they never came and Donald started to raise the boys.

I think everything up to the phone call is by the nephews' original creators. And seeing to how the boys were created as dangerous and totally out-of-control brats, and that the mother was leaving the town...and then commonly visioned as never coming back...It is most likely meant that the parents deliberately abandoned the boys. And since the mother sent them to Donald, apparently they hoped he would raise them. And so he did.