Star Wars: Clone Wars Season 7 Episode 10 - 5 Ups & 2 Downs From "Phantom Apprentice"
5. Up - Revenge Of The Sith
Early in the episode, Ashoka and Obi-Wan have a quick conversation via hologram. Obi-Wan reveals to her that Count Dooku is dead, executed by Anakin as they rescued the Chancellor. He also says he cannot come to help them, since he is headed off to Utapau to hunt General Grievous. Timeline wise, this places the Phantom Apprentice during Star Wars: Episode III - Revenge of the Sith.
The trailers for Clone Wars Season 7 have suggested some overlap with the final film of the prequel trilogy, but only now has it officially started. Though not too major as of yet, confirming that the rest of the season takes place during Revenge of the Sith through dialogue teases some major revelations for the final two episodes.