>>713531915
There is a story about the triplets' mother being alive, but she's an astronaut who's stuck in time dilation due to high speed space travel.
These panels are from a comic where they apparently established contact with her after 10 years, while she's still out there traveling.
DuckTales reboot adapted this story by making Della again a pilot, who crash landed on the moon and had been living there. Scrooge couldn't locate her even after years of trying.