On the contrary, I think DOFP redeemed X3 in a way by implying that everything in that movie led to the dark future. Magneto's war and Phoenix's massacre of Alcatraz Island would absolutely convince a lot of people that the Sentinels are needed, hence the dark future. The President being accepting of mutants and installing Beast as Secretary of State does not automatically mean everything works out in the long run.
I thought it was perfectly clear that Mystique wasn't the sole trigger for the dark future. She was the start sure, but it was the combination of so many events that brought it about.
I don't know why people call it a continuity error when they say that if Sentinels were being developed in 1973, why weren't they unleashed onto the general populace until after the OT. I'm sure perfecting mutant killing robots and pushing the government to greenlight their release, especially when their original creator has died, takes time.
It makes absolute sense why it took that long, especially since it was implied that the Sentinels were perfected not just because they had Mystique's shapeshifting abilities, but Rogue's power absorption as well.