If you have the nerves to wait resolving this and to show up at the airport to let DL or A3 deal with it, I would do it. Print your original itinerary and play dumb at the A3 check-in counter in ATH - the check-in agent notice the cancelled flight and most likely you will be referred to go to the ticket office/desks of DL or A3 to have this solved. At that time the ticket is under "airport control" and airport staff have much more leeway to solve this issue than phone/call center agents. You may get caught in the usual ticketing carrier (A3) and faulting carrier limbo (each one referring you to contact the other) but be firm and ask A3 to deal with this at ATH unless you can find DL staff at the airport prepared to do it.

Anyway, after outbound segments are flown, the carrier is in control of the ticket (and the OTA has no longer anything to do with the itinerary), so you could contact A3 or DL upon arrival at ATH - maybe they are prepared to fix it then (and before you show up at the A3 check-in desks)