Big Three

The Big Three gods are Zeus, Poseidon, and Hades.

Power Levels
The Big Three are the three most powerful gods in all of Olympus, Zeus God of the Sky, Posidon God of the Ocean, and Hades God of the Dead. The Big three are matched in power for the most part, Poseidon commanding the seas which cover most of the Earth, and Hades being the God of Dead representing every person who has ever died. Dispite Poseidon and Hades' large power, Zeus commands the Sky which covers the entire Earth making him the most powerful. Though Zeus is the most powerful of the three Zeus could still, though it would be unheard of, be overthrown by Poseidon or Hades, so he's always watching, just incase Hades or Poseidon try something.

Children
Since the Big Three are the most powerful Gods in Olympus, their Demigod children are the most powerful Half-Bloods. Their Half-Bloods were causing big trouble during World War II, that alone was bad but things got even worse when the Oracle predicted the "Great Prophecy". The Prophecy said that a child of the big three would decied the fate of all Olympus on their 16th birthday. When they heard it Zeus, Posedion and Hades agreed not to have anymore kids from then on, swearing on the River Styx. Dispite them all agreeing not to all of them had atleast one kid since then(with the exception of Hades who had twins). Zeus having Thalia, Poseidon having Percy, and Hades having Nico and Bianca. Percy had been living with his mom until he was 12 and arrived at Camp Half-blood. Zeus ordered Hades to send Nico and Bianca to camp Half-Blood within a week, but Hades wouldn't as he knew that Zeus would be able to get them there. It had been a week since the deadline, and he was stil. 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 then hate him.

Children of the Big Three
As said before all of the Big Three agreed not to have children but Poseidon and Zeus did. Though Hades already had two kids when they swore the Oath, so technically he never broke it. The worst he did was never sending Nico and Bianca to camp Half-Blood.

Nico and Bianca
Hades, already had Nico and Bianca when The Big Three made their agreement not to have anymore 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 mom Maria to let him take them to Hades so he could protect them. Maria repeatedly said no, the last time Hades saw her she was going upstarirs to get something. Zeus then struck the building with a lightning bolt completly obliterating it, Hades sensed the attack and had enough time to put a shield around Nico and Bianca but their mom was killed. It was then that he told one of the furries(Mrs. Doddes) to take Nico and Bianca to the Lotus Hotel. Where they would be hidden until he sent for them.

Thalia
Thalia was the first Half-blood born after the Oath, but she ended up becoming the tree that guards camp Half-Blood for almost 10 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, Clarssie, Grover, and Tyson retrieved The Golden Fleece, from the sea of monsters. However the Fleece worked too well and Thailia 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 Prophecy.

Percy
Percy was last Half-blood to be born after the Oath. He is the son of Poseidon, if it wasn't bad enough that he shouldn't have existed, Percy's presence was revealed at the worst possible time, as Zeus' and Master Bolt his Godly Weapon and symbol of power, and Hades' Helm of Shadows his Godly Weapons and symbol of power, had been stolen. And the appearence of a Son of Poseidon made Hades and Zeus suspicious, as God can' go into each others territory, but if Poseidon had a son he could use him to get the Master Bolt and Helm of Darkness. Percy found both of the items and returned them getting Zeus and Hades to back off. Since then They have stoped hating him and strted to dislike him.