Well, there was the FOURTH nephew, Phooey.... When was the last time you saw HIM?
(Actually Phooey was the name given to the notional fourth nephew who sometimes appeared due to artist error. When drawing three identical characters who usually appeared as a crowd it was sometimes inevitable that a spare arm or leg would appear, or (s)he'd forget that one of them was on the other side of the room. I've also seen this character referred to as "Barks" in honour of Carl Barks)