The General of the droid army in Episode III: Revenge of the Sith is General Grievous.
Grievous is the cyborg commander of the Separatist Alliance’s battle-droid forces during the Clone Wars. He leads the opening assault on Coruscant, commands the flagship Invisible Hand, and takes Supreme Chancellor Palpatine hostage. This makes him one of the film’s major military threats, even though Count Dooku and Darth Sidious are the Separatists’ higher-level political and Sith leaders.
A common mix-up is calling Grievous a droid. He is actually a Kaleesh warrior whose surviving biological body was placed inside a mechanical cyborg frame. His lightsaber skills were developed under Count Dooku’s instruction, which helps explain why he can fight multiple Jedi despite not being Force-sensitive.
By the film’s climax, Obi-Wan Kenobi tracks Grievous to Utapau and defeats him. Grand Moff Tarkin belongs primarily to the later Imperial era, while “General Hax” is not the commander identified in Episode III. Grievous’s official military title is Supreme Commander of the Separatist Droid Armies.