Third Doctor

Now, there was a worthy foe; such cunning, such ingenuity, all wasted through that stubborn streak of goodness!The Master (Destiny of the Doctors)

Biography

Exile

The Third Doctor was the result of a forced second regeneration imposed upon the Second Doctor by the Time Lords. (DW: The War Games) This Doctor was exiled to 20th century Earth and his memory blocked so he could not use his TARDIS to escape. He struck up an arrangement with old friend Brigadier Lethbridge-Stewart wherein he would serve as UNIT's "scientific advisor", and in return could use UNIT's technical and scientific facilities to attempt to make the TARDIS operational again. (DW: Spearhead from Space)

Enter the Master

After the arrival of the Master on Earth, the Time Lords began to allow the Doctor limited use of his TARDIS, but always compelling it to return him to 20th century Earth when his missions were completed, essentially turning him into "some sort of a galactic yo-yo". (DW: The Claws of Axos)

Freedom

When the first three Doctors successfully stopped Omega in his quest to destroy the Time Lords, they rewarded the Third Doctor by restoring his knowledge of the TARDIS codes and lifting his exile. (DW: The Three Doctors)

Guilt and redemption

The Third Doctor continued to assist UNIT, though his presence on Earth was now much more intermittent. Finally, to defeat the Spiders of Metebelis III, he sacrificed his own life by exposing himself to lethal levels of radiation. With the assistance of his old mentor K'anpo Rimpoche, he was able to regenerate into his fourth incarnation. (DW: Planet of the Spiders)

Characteristics

The Third Doctor often had problems with the Brigadier's tendency to think of situations in military terms and with petty officials generally. The Third Doctor had a continuing series of contests and challenges with the Master, and was the first known incarnation to encounter the Silurians and Sea Devils as well as the Ogrons, and the Autons. (DW: Doctor Who and the Silurians, The Sea Devils, Spearhead from Space, Terror of the Autons,Day of the Daleks)

Psychological profile

Personality

The Third Doctor was a man of action, aggressively joining the fray whenever he could, unlike the first two Doctors who generally insinuated themselves into events discreetly. This was a Doctor was unafraid to pitch in with his physical skills, often bringing his mastery of Venusian aikido into play when the situation called for it.

But much like his predecessors, his keen mind was still his primary asset, and this was a Doctor who particularly loved to create and play with gadgets of all sorts. This passion displayed itself both in terms of the Third Doctor's scientific bent and in his love of vehicles, such as his yellow roadster Bessie and his car which he specially built.

Staunchly moral, often to the point of being preachy, the Third Doctor was every bit the gentleman, a hero of the Victorian mould.

Habits and Quirks

The Third Doctor was known for his love of gadgets and a passion for his vintage car Bessie and later his futuristic Whomobile. He was the most action oriented, using a wide variety of martial arts including Venusian Akido.

Mysteries and Discrepancies

The Doctor sported a tattoo on one arm, which seems impossible given he has just regenerated. (DW: Spearhead from Space)

The Doctor probably had not a conventional tattoo, but the mark of an exiled Time Lord. (NA: Christmas on a Rational Planet)

The Doctor describes himself as thousands of years old (DW: Doctor Who and the Silurians), which seems inconsistent with figures of his age given in later incarnations.

He repeats that age later to Jo Grant. Previous incarnations of the doctor gave his age to "some hundred years" (The first doctor gives even a distinct number in the range of 700-800y). Although he could just be meaning how long ago he was born (The date of his birth compared to the date he meets the Silurians, not his actual age)

Unrecorded adventures

Though sidetracked to the planet Nooma (MA: Speed of Flight), the Doctor, Jo and Mike Yates visited the planet Karfel and encountered the Borad and Katz's father there. (DW: Timelash)
In Castrovalva, the Fifth Doctor mentions being with the Brigadier and him chasing Ice Warriors, so this must have been an unrecorded adventure for him.

Key Life Events

Newly regenerated, partially amnesiac and exiled on Earth, the Doctor re-unites with UNIT and the Brigadier and encounters the Autons and Nestene Consciousness for the first time. Afterwards, having nowhere else to go, the Doctor reluctantly accepts the position of UNIT's unpaid scientific advisor. Is given his own automobile, which he dubs "Bessie", and Dr. Liz Shaw becomes his assistant. (DW: Spearhead from Space)
Is briefly sent to a parallel earth. (DW: Inferno)
Liz Shaw leaves UNIT and Jo Grant becomes the Doctor's new companion. The Doctor also meets Captain Mike Yates for the first time. (DW: Terror of the Autons)
A Time Lord warns the Doctor that a former friend turned enemy, the Master, has come to Earth. (DW: Terror of the Autons)
Encounters the Daleks for the first time in many years; also, due to a TARDIS malfunction, accidentally meets a future version of his same incarnation. (DW: Day of the Daleks)
With his first and second incarnations, the Doctor defeats Omega. As a reward for this, the Time Lords rescind his exile and restore his knowledge of the TARDIS. (DW: The Three Doctors)
With his restored freedom, the Doctor begins scaling back his involvement with UNIT and starts travelling through time and space again. (DW: Carnival of Monsters)
The Doctor emotionally says goodbye to Jo Grant, who leaves in order to marry an environmentalist. (DW: The Green Death)
The Doctor meets Sarah Jane Smith for the first time. (DW: The Time Warrior)

The above episode also saw the introduction of the Sontarans, however a later serial, The Two Doctors, established that the Second Doctor had previously met the creatures, making this no longer a first encounter. This serial is also the first time the name of Gallifrey is mentioned on screen.

In a timeline created by Faction Paradox, the Doctor dies on the planet Dust. (EDA: Interference: Book One, Interference: Book Two).
The Doctor re-unites with K'anpo Rimpoche, a spiritual teacher and Time Lord, the Doctor had known during his youth on Gallifrey. (DW: Planet of the Spiders)
The Doctor regenerates. (DW: Planet of the Spiders)