In Christianity, the holy trinity is three deities in one. In Hinduism, Brahman is the main god, with the other gods being a different form of her. The Father, the Son, and the Holy spirit are all forms of God.

Back in Ancient Greece, their polytheism had many different levels of deities, some, like Heracles, Orion, Helen, and Achilles, were demigods. They were humans, but were given a special place in the belief system because of their accomplishments. This is similar to the Catholic Saints, people pray to them because they feel that the Saints are closer to the gods. In Ancient Greece, people would pray to Heracles and set up temples to him.

And last, Satan, he is essentially the god of Hell, even though he's not "all loving" like God, he still plays a significant role in the dynamics of the religion so he is a god. Again, in Ancient Greece, the gods weren't always forgiving, they would punish people for wrong-doing. Even though they weren't "nice" like a god is supposed to be, they're still gods.

Christianity is a polytheistic reason because of it's worship of many gods as explained above.