A History of the Persian Empire
According to the book from Pierre Briant, as revealed on citation 9 on Wikipedia, From Cyrus to Alexander: A History of the Persian Empire there is absolutely no debate to be had on who the pharoah of the time was, with that being Psamtik III.
Conquest of Egypt and its surroundings, Wikipedia
By 526 BC, Amasis II had died, and his son Psamtik III had succeeded him, thus weakening Egypt's position. In the meantime, Cambyses had made substantial preparations for his army. He had essentially laid the foundations to the Persian navy, which was crucial to his ambitions to conquer Egypt.
Psamtik III, Photo by Juan R. Lazaro
Now, according to Clayton 1999, from Wikipedia citation 2, there is absolutely no way Amyrtaeus could have been the pharoah which Cambyses II removed, as they are a completely different time period, and Amyrtaeus is credited with avenging the defeat and removing the Persians from power in Egypt in between 404 BC and 309 BC.
Amyrtaeus or Amyrtaios (both Hellenizations of the original Egyptian name Amenirdisu) of Sais, is the only pharaoh of the Twenty-eighth Dynasty of Egypt1 and is thought to be related to the royal family of the Twenty-sixth Dynasty (664–525 BC). He ended the first Persian occupation of Egypt (i.e. the Twenty-seventh Dynasty: 525–404 BC)
Papyrus dating to Amyrtaeus, by Eduard Sachau (1845 - 1930) -
Now, just to be sure, without any ifs or buts or maybes that most recognised reputable sources do recognise Psamtik III as definitely being the pharoah of the time, i shall provide confirmation from the Britannica encyclopedia.
Cambyses II, Britannica encyclopedia
The conquest of Egypt, planned by Cyrus, was the major achievement of Cambyses’ reign. The invasion took place during the reign of Psamtik III.
What pharaoh did Cambyses defeat in the invasion of Egypt? The current understanding that would likely be accepted on most exam papers would be that Psamtik III was most likely the pharoah removed by Cambyses II, and Amyrtaeus was probably the pharoah which reversed this defeat over 100 years later.