Not a dev, but as you said, it was previously said that the 3 characters would be required for the true story mode (a.k.a. the Nightmare Legacy). Given the first character we got for a long time was Annie who has nothing to do with it, seems to indicate that it's not that high of a priority, although we did get Umbrella as the second character.
I think it is entirely possible that the Nightmare Legacy would be implemented in a completely different way than it was originally planned, if at all, given the change in the general situation surrounding the game. I doubt it's something the devs are willing or able to confirm at this point, since their primary priority now is likely to deliver the Season Pass characters on schedule and getting the patches out for all supported platforms.
The next 2 characters chosen could be a good signal for how they are looking to go with this. But then again, even if Dahlia and Marie were chosen, it might not mean anything at all. They are pretty popular characters on their own regardless of inclusion in the canon story.