Big Three

For the pact of the Big Three, see here.

The Big Three is a collective term which refers to the three major Olympian gods: Zeus, Poseidon, and Hades.

The Big Three
The Big Three are the three most powerful gods in all of Olympus: Zeus, Lord of the Sky, Poseidon, Lord of the Seas, and Hades, Lord of the Underworld. The Big Three are equally matched in power for the most part, with Poseidon commanding the seas which cover most of the Earth, and Hades being the God of Dead, representing every person who has ever died and now is a ghost. Despite Poseidon and Hades' large power, Zeus commands the sky which covers the entire Earth, but has few inhabitants, and even fewer are sentient, while Poseidon and Hades have many more creatures in their domains, both non-sentient (fish) and sentient (the dead, naiads, and mermaids). Despite this, as stated in myth and The Lost Hero, Zeus is generally considered the most powerful god partially thanks to his control over the sky which is above all and partially thanks to holding the most powerful weapon, the Master Bolt.

The Earth (Gaea) is split between the gods and is no one's specific domain, though Zeus is initially believed to control the earth. However that is debatable since Demeter is Goddess of the Earth as well. Also the other gods are not to be underestimated because the Big Three only get their raw power from how high they are among the Olympian hierarchy.

Children Of the Gods
The Big Three are the most powerful gods in Olympus, therefore their demigod children are the most powerful Half-Bloods. The Half-Bloods were causing big trouble during World War II, with the sons of Zeus and Poseidon on one side and Hades' sons on the other. The final straw came when the Oracle predicted the Great Prophecy. The Prophecy said that a child of the Big Three would decide the fate of all Olympus on their 16th birthday.

When they heard the prophecy, Zeus, Hades, and Poseidon agreed not to have any more kids from then on, swearing on the River Styx. This was later called the Pact of the Big Three.

Despite them all agreeing not to have any more demigod children (with the exception of Hades who already had two children before the oath), Zeus "fell off the wagon" and had Thalia, while five years later Poseidon sired Percy Jackson. Hades having Nico and Bianca did not go against the rules of the oath since they were born before the oath was made, but were frozen in time in the Lotus Hotel and Casino. It's also hinted that Poseidon might have had more children in the fifth book, The Last Olympian, but he could have been joking. In The Sea of Monsters, it is also found that Poseidon had cyclops children such as Tyson, considered mistakes between gods and nature spirits.

Percy had been living with his mom until he was 12 and arrived at Camp Half-Blood. During World War II Zeus tried to kill Bianca and Nico by obliterating the whole building they were in with a lightning bolt, killing their mother and Hades' lover, Maria di Angelo. After that Nico and Bianca mysteriously disappeared until they were found in The Titan's Curse. All the children of the Big Three are hated by the other two gods; Percy was hated by Zeus and Hades in The Lightning Thief, though they have grown to dislike him rather than to hate him. Hades hates Thalia and tried to kill her, but in the end Zeus spared her and turned her into a pine tree.

At the end of The Last Olympian, Percy relieves the Big Three of their oath.

Prophecy
''A half-blood of the eldest gods Shall reach sixteen against all odds<brAnd see the world in endless sleep The hero's soul, cursed blade shall reap A single choice shall end his days Olympus to preserve or raze

Children of the Big Three
As said before, all of the Big Three agreed not to have children, but Poseidon and Zeus did. Hades already had two children before they swore the oath, so technically he never broke it. The worst he did was never sending Nico and Bianca to Camp Half-Blood. Poseidon has Percy Jackson and a cyclops named Tyson, however because Tyson isn't a demigod, there is no fear of him fulfilling the first Great Prophecy. Zeus had a child named Thalia Grace. Zeus technically only has one living child like Hades, which is Thalia because Jupiter (Zeus's Roman form) had Jason in his Roman form, not his Greek form.

Nico and Bianca Di Angelo
Hades already had Nico and Bianca when the Big Three made their agreement not to have any more kids, for fear of the Great Prophecy. Zeus gave Hades a week to send them to Camp Half-Blood, but Hades never did as he knew that Zeus would destroy them the minute they were out of Hades' territory. It was a week after the deadline and Hades was still trying to convince their mother Maria to let him take them to the Underworld so he could protect them. Maria repeatedly said no, and the last time Hades saw her, she was going upstairs to get her purse. Zeus then struck the building with a lightning bolt, completely destroying it; Hades sensed the attack and had enough time to put a shield around Nico and Bianca but Maria was killed. It was then that he told one of the Furies; Alecto, to take Nico and Bianca to the Lotus Hotel and Casino, where they would be hidden until he sent for them. When they were found, Bianca joined the Hunters of Artemis while Nico went to Camp Half-Blood. However, Bianca died to save Percy and his friends from a defective version of Talos, leaving Nico as the last child of Hades alive.

Thalia Grace
Thalia was the first demigod born after the Oath, but she ended up becoming the tree that guards Camp Half-Blood for almost seven years. She was revived in book two, The Sea of Monsters, when her tree (the source of the magical border around Camp Half-Blood) was poisoned by Luke. Percy, Annabeth, Clarisse, Grover, and Tyson retrieved the Golden Fleece from the Sea of Monsters. By mortals, it is known as the Bermuda Triangle.

However the Fleece worked too well and Thalia was revived from the tree. She later joined Artemis' Hunters, meaning that she would not age and thus couldn't be the Half-blood in the Great Prophecy.

Thalia had a Roman younger brother named Jason, but he was abandoned by his mother, and taken by Lupa to go train to become a warrior at Camp Jupiter.

Percy Jackson
Percy was the second child of the Big Three to be born after the Oath. He is the son of Poseidon and Sally Jackson, and originally did not know anything about his godly heritage. He is probably the most powerful demigod in Camp Half-Blood like Jason, Thalia, Nico, and Bianca. Percy's presence was revealed at the worst possible time, as Zeus's Master Bolt, his symbol of power, and Hades's helm of darkness had been stolen. The appearance of a Son of Poseidon made Hades and Zeus suspicious, as gods can't go into each others' territory, but their children can.

Percy found both of the items and returned them, successfully preventing a war. Since then, Zeus and Hades have stopped hating him and now merely dislike him. It is also implied that the Big Three might have more children, as stated in The Last Olympian, but they are now allowed to have children.

Trivia

 * Chiron mentioned that "Three" is a holy number, as it is the number of the Big Three, the three Furies, three Fates and the three virgin goddesses.
 * During the mortal World War II, the Allies which represented the British Commonwealth, the Union Soviet of Socialist Republics, and the United States of America were known as the "Big Three." :
 * Winston Churchill, son of Poseidon of the British Commonwealth.
 * Adolf Hitler, son of Hades.
 * Franklin Delano Roosevelt, son of Zeus of the United States.
 * The 'big three' is also used as a collective name for the three major allies of the mortal World War I; David Lloyd George, George Clemenceau, and Woodrow Wilson. Though it is currently unknown whether they are demigods.