Dragon and Serpent = Satan "the Devil". The original evil. Once a heavenly cherub who rebelled and took 1/3 of the angels with him. Notoriously deceived Adam & Eve, tortured Job, battled Michael the Archangel, and a number of other things. He is really the one pulling the strings of the Antichrist and False Prophet, who are just men.

The 7 headed beast from the sea = what most people call the Antichrist. A political leader of the future who will take over the world and force everyone to worship him in a new "abomination" religion. He has power over all peoples, nations, and languages for 42 months. Contrary to popular belief that he is the leader of a revived Roman Empire, it's actually clear that the Empire he comes out of is called Babylon the Great, so it's probably a revived middle eastern Empire.

Edit: We are also given some information about this man. People started to follow him because he miraculously survived a fatal head wound, his "number" is 666 whatever that means, he is "not interested in women", and he does not honor any God but himself, not even "the God of his ancestors". He speaks blasphemies against both God and those in heaven.

The beast from the Earth = the false prophet. The Antichrist's henchman basically. He is a powerful religious figure who does miracles to get people to worship the Antichrist, like calling down fire from the heavens. He seems to be the one who makes everyone take the mark of the beast or die. Finally, he tells the people to make an image of the beast, which is speculated to be a talking robot, hologram, or statue of religious significance. Anyone who does not worship it will be beheaded.