Can I have a summary of The Halo story up to Halo 3?

I have just purchased Halo 3 and I would like to know the story up to it so that I can play the campaign and understand what is going on.

Solution 1:

Here is the review of the games up to Halo 3.

1. You wake up
2. Covenant attack the ship you are on
3. The general for the Pillar of Autumn tries to land on the Halo ring
4. You get out of the ship on an escape pod
5. You crash land on ring
6. You search for others who used pods
7. Go into the facility to find the Halo control room
8. Covenant accidentally release Flood
9. Work with guilty spark to kill Flood
10. You figure out Guilty Spark is trying to turn on ring
11. You self destruct the crashed Pillar of Autumn to destroy the ring

1. It shows a Covenant Elite commander being stripped of his rank and then tortured by Tatarus, a Brute warrior
2. The Chief, Johnson, and soldiers are awarded for their actions on the first ring
3. A Covenant fleet appears outside of Earth
4. They attack New Mombassa, Kenya.
5. Chief takes them out, and the Covenant retreat through slipspace
6. The Chief fallows in the In Amber Clad
7. They find another ring, and chief goes to destroy it
8. The disgraced Covenant commander is offered position of the Arbiter
9. He goes to kill a heretic, and finds 343 Guilty Spark (Oracle)
10. The Covenant go to the newly found ring due to a distress signal from the Prophet of Regret
11. Tatarus tells Arbiter that the Prophets plan on annihilating the Elites
12. The Arbiter is sent down a chasm
13. The Gravemind saves him, and the Arbiter meets the chief
14. The Gravemind tells Arbiter that the Great journey will wipe out the Covenant, Humans, and the Flood
15. The Chief teleported to High Charity where a civil war is happening
16. Cortona realizes that the Gravemind used them as a distraction to infest the In Amber Clad
17. The Gravemind crashes it into High Charity to spread the Flood
18. Chief goes onto Forerunner ship and leaves Cortona behind to destroy High Charity and the Halo ring
19. Arbiter goes to the ring and rallies his allies to fight the brutes
20. Arbiter and Johnson kill Tartarus
21. They turn off the ring, but instead it makes it so all rings can be activated from the "ark" at a safe distance
22. Chief goes back to Earth

Solution 2:

Background Story

Humanity has expanded into space, colonizing many planets in different star systems. Humanity is still the dominating species in the galaxy until one colony is attacked by a group of aliens called The Covenant.

The Covenant is a hierarchical group of different alien species. They view humanity as a plague and are waging a holy war against them with the goal to eradicate humanity from the galaxy.

The war goes on for many years and humanity is on the losing end, mainly because The Covenant has superior technology.

Before The War, a top-secret organization had started to create super soldiers. Young children who trained their entire youth and have been augmented to be physically superior to normal humans. This project was named Spartan II (2)

You play as one of the last of these super soldiers, John-117.
You are accompanied throughout the majority of the games by one of the top A.I.'s in human history, Cortana.

Halo CE

The Covenant find and destroy the planet Reach, one of the biggest military installations of humanity and home to the Spartan II project.
Your ship, The Pillar of Autumn, has escaped from Reach by making a blind lightspeed jump into space.

Upon exiting slipspace, your ship is at an unknown alien installation (the Halo Ring).
The Covenant is already at this installation because they have already discovered it. The Covenant engages your ship and shoots it down.
While the ship crash lands on the alien installation, most personnel, including yourself, is evacuated with escape pods and pelican dropships.

After crash-landing on the Halo ring, you start a search and rescue to regroup with other human allies and stage a rescue assault on a Covenant battle-cruiser to save the Captain of The Pillar of Autumn, Captain Jacob Keyes.

During this time, you learn that the Halo ring is a weapon and that it is not built by The Covenant. The Covenant are however trying to activate the Halo Ring to use against Humanity.
You and the rest of the survivors start a search to reach the control room of the installation before The Covenant does.

After finding the control room, Cortana learns that the ring is not a weapon but something more dangerous. She sends you to find and stop Captain Keyes, who is at that time looking for a weapons cache.

While searching for the Captain, you encounter a new enemy, The Flood. The Flood is a parasitic life form, that kills all viable life forms and then reanimates and transforms the corpse into a combat form to attack and obtain more bodies (vessels for spores).
The Flood was contained in special facilities on the Halo Ring and is the weapons cache that the Captain was looking for.
The Covenant has, however, already released The Flood.

While escaping the facility, you encounter an alien A.I. named 343 Guilty Spark. He explains to you that the ring is a Forerunner construction and that he is the monitor of Installation 04 (the Halo Ring).
343 Guilty Spark informs you that The Flood must be stopped and that the only way to contain the outbreak is to activate the Halo Ring. You assist Guilty Spark in retrieving the activation key for the Halo Ring.

When you return to the control room with Guilty Spark and the activation key, Cortana informs you that the ring is designed to wipe out all sentient life in the galaxy. This is the only way to stop the flood, by starving them.
Cortana steals the activation key to prevent Guilty Spark from activating the ring.

You are now being hunted by 343 Guilty Spark and his Sentinals whilest The Flood is spreading throughout the Halo Ring and trying to find a way to leave the installation. The Covenant is trying to evacuate the Halo Ring.

You continue your search for Captain Keyes, because Cortana has a plan to blow up the remains of the Pillar of Autumn so that the Halo ring will be destroyed and cannot be used to wipe out all sentient life.
You eventually find Captain Keyes on a stranded Covenant cruiser which is being consumed by The Flood. Captain Keyes is also consumed by The Flood. You take his neural implant to obtain certain access codes and head to the crash site of The Pillar of Autumn.

You destroy the engine of The Pillar of Autumn and escape the ring in a small space fighter called a Longsword.
The Halo ring is destroyed, 343 Guilty Spark survives the destruction and is seen flying into space.

Halo 2

Prophets: the main leaders of The Covenant.
Elites: the leading military race within The Covenant.
Brutes: the newest race to join the Covenant, has no leading ranks but has enormous fighting power.

Halo 2 starts with a trial in the Covenant's "mother ship" called High Charity.
A Covenant commander is accused of heresy because he failed to protect Installation 04 (The Holy Ring) from the destruction by the hands of the Demon (John-117).
He is stripped of his rank and sentenced to death, The Prophets give him the option to become The Arbiter, a suicide unit sent out to complete tasks to maintain order within the Covenant.

A small Covenant fleet finds Earth, one ship breaks through the space blockade and lands on the continent of Africa.
You are sent to assault the Covenant Carrier that landed in an attempt to capture a Prophet. While starting the assault, the Covenant ship jumps into slipspace, in a desperate attempt, you follow it on the ship In Amber Clad with Captain Miranda Keyes (daughter of Jacob Keyes).
After exiting slipspace, both ships are at another Halo Ring, Installation 05.

The Arbiter is sent to find and kill a resistance leader within the Covenant. They find this leader at an installation near the destroyed Halo ring, Installation 04.
The resistance leader had found 343 Guilty Spark and calls it The Oracle, Guilty Spark has told the resistance leader about the Halo Rings and their purpose.
While the Prophets and The Covenant still believe the rings to be their holy weapons, this small resistance knows the truth.
The Arbiter kills the resistance leader and brings The Oracle to High Charity. High Charity and the entire Covenant fleet then enter slip space and go to the newly found Halo Ring.

John-117 lands on Installation 05, find the Prophet that escaped earth and kills him before he can active the Halo Ring. The Covenant that just arrived with their entire fleet retaliate by bombing the site that John-117 is currently at. John-117 falls into the deep water of the Installation and is found (and rescued) by The Gravemind.

Because of the death of the Prophet, the Elites are removed from military power,Tartarus and the Brutes assume command of the militairy forces of the Covenant.
The Arbiter is sent onto Installation 05 to find the Activation Key, needed to activate the Holy Ring.
The Arbiter fights his way through heavily flood infested areas to retrieve the Activation Key. Human forces have also fought their way to the Activation Key. The Arbiter finds the activation but is betrayed by Tartarus.
The Arbiter is thrown into a cliff as he is told by Tartarus that all the Elites will soon be purged from The Covenant. The Arbiter is found by The Gravemind and a civil war starts in the Covenant.
Tartarus takes Captain Miranda Keyes hostage.

The Gravemind is the hivemind for all the flood on Installtion 05, it has consumed another Prophet and the Monitor of the installation, The Flood is clearly dominant on the Halo Ring and far more advanced then it was in Halo CE's outbreak.
The Gravemind tells The Arbiter the truth about the Halo rings, the opens the eyes of the Arbiter about their Holy War against humanity. The Gravemind teleports The Arbiter to the location of the control room of the installation and teleports John-117 to High Charity.

The Arbiter finds allies and starts to make his way towards the control room, on his way he meets up with Sgt. Johnson and form an alliance to stop Tartarus from activating the Halo Ring. Together they breach the control room and kill Tartarus.
Tartarus however did activate the Halo Ring, but Captain Keyes was able to stop the activation sequence.
This means that all the Halo rings are on stand by and can only be activated from The Ark.

John-117 fights his way through High Charity in an attempt to stop the Proghet of Truth of escaping.
The Prophet boards a Forerunner ship and escapes High Charity, John-117 boards the ship in the very last second before its departure. The Prophet is heading for The Ark

At the same time, The Gravemind has assaulted In Amber Clad and consumed it.
He crash landed it on High Charity to spread his infection, Cortana stays behinde on High Charity to destroy In Amber Clad and stop the spread of the infection.

John-117 promises Cortana that he will come back for her, Cortana tells him not to make promises he cannot keep.

The Prophet of Truth and John-117 arrive back on Earth.

Note:

I know this answer is 4 years late, but the current answer is rather short and is not always clear on the identities of characters or locations without having played Halo 3 already. My answer aims at people who have not yet started Halo 3. I am also aware that my answer is quite long to be "a summary". But now there are 2 answers to provide information a long one and a short one.