Mythology of Scorpio, Leo, and Cancer
There are many variants of the mythology of Scorpio, but nearly all of them also include the constellation of Orion, the Hunter. In some versions, Scorpio was an assassin sent by Artemis to kill Orion. The reasons vary. Sometimes, it's out of rage at her defeat at his hands. In another, it's to protect the bull of Taurus, and in another, it's for his bragging of his skills and glory in hunting (Artemis believed in hunting only for food). And in yet another version, the scorpion was a punishment for Orion's pursuit of the goddesses of the Pleiades.
Regardless of the version you like to believe, the assassination of Orion plays out again and again every night in the stars. Orion is opposite Scorpius in the sky, so as the scorpion arrives on the scene, Orion "dies" by dropping below the horizon.
Leo and Cancer both owe their existence--and their deaths, unfortunately--to the great Olympian hero, Hercules.
Hercules was charged with killing the vicious and giant Nemean Lion. None of his weapons could cut through the lion's skin, so he was finally forced to wrestle it to the ground and strangle it with his bare hands.
Hercules kept the lion's skin, and it came in very handy. His next task was the Lernean Hydra, a dragon with seven heads. And, as if multiple heads weren't enough of a problem, when any head was cut off, two more would grow in it's place! The Lion's skin helped protect Hercules from the heads while he set about to find a way to kill it.
Hera, Zeus's wife, wanted Hercules dead. She sent a crab to distract Hercules from his fight with the Hydra. Unfortunately, Hercules was only distracted long enough to crush the crab under his foot. Hera placed the crab in the sky to remember it's sacrifice.