This sounds a lot like /r/homeworkhelp territory. Nevertheless, to get you started on your essay, let's try this:

Herodotus, the father of history, is given to exaggeration and anecdote. His stories are fantastic and appear to be an effort to bridge Greek mythology with Greek history.

Thucydides, commonly called the first historian in contrast to Herodotus, is dry, obsessively concerned with attribution and detail, and prone to giving as even-handed a take as he can regarding the actions of the early Peloponnesian War. His speeches tend to evoke philosophical questions about the nature of war and power (see the Melian dialogues, for instance, or Pericles' funeral oration).

Xenophon is something of a grab-bag. I recommend consulting the foreword to the Landmark edition of the Hellenica for a comprehensive and readable summary of Xenophon's style and influences.

Finally: if you're in a history class, and you were assigned these texts, it's a real shame you didn't read them. I sincerely believe any person could benefit- in the way they orient their lives- from the study of Greek history under the tutelage of these three great historians. So, maybe the answer to your question is, RTFM.

I've read bits of all of them in the original so I feel that I can compare and contrast to some extent.

Herodotus to me reads something like a prose epic with lots of drawn out similes and metaphors and vocabulary that could be drawn from the Iliad or Odyssey. It is generally believed that the histories were first intended to be orated by the author himself in the same manner as a recitation of Homer. He does not always tell us exactly what he thinks happened, but rather gives us two different versions of what might have happened, allowing the reader to make up his own mind what happened. As /u/HMSErebus has noted his stories are often fantastic involving gryphons and humorous ways of collecting gold. It is interesting in and of itself that it is not written in Attic, the dialect spoken by the Athenians, but rather Ionic and as such has some odd changes to vowels and tenses being somewhat dissimilar to Attic.

Thucydides is very down the line and prose is very lucid and clear although not without technical difficulties. He is particularly interesting in that he banishes the gods completely from his history. There is no mention of them apart from when the Greeks make sacrifices to them. However they have no agency. Quite tellingly he gives an almost anthropological reason for the emergence of theHellenes and the Athenian empire. He puts it down to the migrations of tribes on the one hand and the poor quality of the Athenian soil on the other, forcing them to work together.

Xenophon I personally find very, very dull. However, he is incredibly straightforward, at least in the Hellennica and the Anabasis. Basically one long string of participles after another. To me, it seems very pious and Conservative, celebrating the end of the journey with a set of games. Also, the way that it is written makes it seem as if he had command thrust upon him because of his sheer genius, which rings a bit hollow to me. But we only have his word to go on so.... The apology is actually quite interesting and gives a very different impression of Socrates than the one we are given by Plato.

If I have the time in the morning I will upload some translations with the original text and point out interesting things.