Originally Posted by
Mokshu
I added the fact that even if the third segment is closed for check-in, you should be able to do it as first segment is a connection flight.
I doubt whether this is correct but I have insufficient information provided to me about the Deltamatics system of Delta (in which I assume the 3rd sector is handled).
What is true is that when checking in online, flights open 30 hours (24 for US flights) before the
first sector and you can check-in subsequent sectors too, so quite some hours sooner than passengers without connecting flights -but usually only if the flight is
open for check-in in the respective DCS.
Sometimes >30 hours before departure they are still preparing the flight meaning check-in is inhibited.