According to the folks at Windows Phone Central, they claim that the screenshots are legit and that this is the real deal. The device appears to be running on firmware based from India and based on the screenshots, it looks like the GDR3 will play host to a bunch of new features, not just supporting hardware upgrades to Full HD displays and quad-core processors.
These features include a screen orientation lock, a driving mode that will ignore calls and texts to prevent distraction while driving, custom sounds for text, voice mails, emails, and reminders, and more. No word on when Microsoft is expected to debut Windows Phone GDR3, but it is speculated to be unveiled alongside the upcoming Nokia Lumia 1520.