In my book on Ancient Civilizations in the Ancient Persia section, there is a mentioning of three Indo-Iranian tribes that came together to form the empire, but I don't think they're mentioned again in the book, and the internet hasn't really given me a definite answer so far so could someone give me some sort of clarification on what these three tribes were called? Were they the Achaemenid Empire, Parthian Empire, and Sassanid Empire, or is there something I've missed? Thanks in Advance.
From the sound of it, I would think they are referring to the Persians, the Medes, and the Parthians. All three were West Iranian. Before the campsigns of Cyrus, the Persians had absorbed much of the older, more urbanized Elamite cultures, whereas the Medes, a tribal federation living more to the north, possessed remowned cavalry. Cyrus united these two tribes before going on to lay the foundations of his empire.
The Parthians only really come into play around 250 BC, some 70 years after Alexander's death, when Aršak, a chieftain from some Central Asian tribe, invades the satrapy of Parthia (today NE Iran/Turkmenistan). While remaining nominally loyal to the Seleukid Basileus, his successors would eventually go on to conquer most of the Seleukid empire, leading to the Parthian or Arsacid empire, which was in general heavily Hellenized.
The Sasanian Empire directly follows the Parthian, when in the 3rd century the Persian vassal king Ardaxšir (claiming descent from an obscure figure named Sasan) begins a rebellion and not only conquers the Arsacids but, along with his successor Šapur, goes on to reconquer vast swathes of vital territory in Mesopotamia and India, and humiliates the Romans. By this time, the distinction between Medes and Persians is lost, but due to the prominence of Parthian nobility, that distinction would linger for some centuries to come.