Name 2 social, cultural, or political characteristics among the poleis of Athens, Sparta, and Miletus.

It's important to recognize that Classical Greece comprises a much larger geographic expanse than the modern nation of Greece. Note that Athens and Sparta are located in the modern country of Greece, with Sparta situated in the Peloponnese and Athens situated in Attica, as compared to Miletus, which is located in Asia Minor.
I would note that both Athens and Sparta shared in a great hatred of tyranny. By this, I mean tyranny within the Greek context, in which Greek aristocratic politics would fall under the control of a single dominant personality (For more information, see Ian Morris & Barry B. Powell, The Greeks: History, Culture and Society: Upper Saddle River: Pearson, 2006, chapter 8). In both Athens and Sparta, political structures evolved to limit the concentration of power. In the Athenian case, there was ostracism and the use of sortition (by which offices are chosen by random selection). Both served as means to prevent individuals from building too much influence (see Morris & Powell, chapter 10). Meanwhile, one can look at Sparta, with its divided Kingship (there were always two Kings) and the supervisory powers of the Ephors. Indeed, there is an argument that, in the case of Sparta, "different institutions exercised checks and balances on each other. The kings controlled war and religion, the Elders controlled law, and the Ephors fair play." (Morris & Powell, 293)
Additionally, it's worth noting that while Sparta was the most militarized state in Greece, its militarism was largely defensive in purpose, and historically, it's foreign policy was focused largely around the Peloponnese. The Athenians, on the other hand, would become far more ambitious and aggressive, taking a leadership role in Delian League (an alliance aimed against Persia), which it would later convert into empire. This empire would, in turn, fuel the Athenian Golden Age.
Finally, to turn towards Miletus, it's worth noting that Miletus, for part of its history, fell under the control of the Persian Empire. In addition, Miletus was one of the earliest centers of Greek philosophy and was home to critical thinkers such as Thales, Anaximander, and Anaximenes. This was a key intellectual center during the early history of Greek philosophy, and Athens would emerge as a key intellectual center much later within that history.

While there are certainly similarities between the poleis of Sparta, Athens, and Miletus, there are also some important differences. An interesting social characteristic of Sparta was the fact that the city-state was purely militaristic; a higher military rank signified a higher social status. While it was generally a male-dominated society, women in Sparta had a bit more rights in comparison with the other city-states. Another interesting characteristic is that Sparta actually had two kings who came from two different powerful and aristocratic families.
Athens was economically and politically the most developed society among the Greek city-states. The people of the polis were wealthier in comparison, because they were included in the trade business. Athens also had a male-dominated democratic political system, and the citizens spent their wealth on building various structures and houses, which were considered the most beautiful in all Greece.
Miletus didn’t really have a strong militaristic power like Sparta or a well-developed political system like Athens, but it did hold great commercial power, as the people focused on the trading business. Miletus was probably the only polis where the people heavily concentrated on art, culture, philosophy, and science.