Third Dalek War

The Third Dalek War was a war fought between the empires of Earth and the Daleks. (PROSE: The Chase) It was one of the conflicts of the Dalek Wars. (PROSE: The Secret Lives of Monsters) The final action of the war was the Exxilon Gambit, (PROSE: The Secret Lives of Monsters) identified by the Time Lords as taking place in the late 27th century. (PROSE: Dalek Combat Training Manual)

Origins
The Third Dalek War took place around the same period as the Human-Draconian War, (PROSE: The Chase) and the Second Dalek War. (PROSE: Deceit) The latter conflict ended circa the 2580s. (PROSE: Prisoner of the Daleks) The Galactic Federation Archivist and historian Ven Kalik wrote that the conclusion of the Second Dalek War was "untidy" and the conflict ultimately "fizzled out", but "another Dalek War" broke out in the period of its aftermath. (PROSE: Deceit)

Human historians postdating the Last Great Time War indicated that the Daleks' germ warfare campaign which culminated with the Exxilon Gambit came multiple centuries following the 26th century conflicts. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

Fighting
By the time of the Third Dalek War, the location of the Lucifer star system was of paramount strategic importance, but neither the Daleks nor the Earth Alliance could go anywhere near it as there was a total exclusion zone, a kind of force field generated by the black holes worshipped by the Angels. IMC wanted Ace to time travel to around 2154 to gather intelligence about it. (PROSE: Lucifer Rising)

The Daleks resorted to biological warfare, (PROSE: The Secret Lives of Monsters) which they had previously experimented with during the Spiridon campaign at the start of the last Dalek war. (TV: Planet of the Daleks, PROSE: Remembrance of the Daleks) Utilising plague missiles, the Daleks infected the Earth's Outer World colonies with a deadly outbreak of space plague, (PROSE: The Secret Lives of Monsters) threatening millions of lives, although the Earth forces seemed unaware the Daleks were responsible for the plague. (TV: Death to the Daleks) Indeed, the wider Earth Empire was unaware of any active fighting against the Daleks, with some citizens even coming to believe the exterminators did not really exist and were simply propaganda used to scare citizens into obeying their governments. The outbreak of the plague only exacerbated that skepticism. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

Conclusion
The plague led to the Exxilon Gambit, the last act of the war. (PROSE: The Secret Lives of Monsters) Parrinium was a mineral capable of curing the plague, but it was extremely rare and only found in abundance on Exxilon. Earth dispatched a group from the Marine Space Corps to the planet to acquire the mineral. The Daleks, as a countermeasure, dispatched an expedition of their own. Both parties became stranded on the planet when the power to their ships was drained by the Great City of the Exxilons. (TV: Death to the Daleks)

With the added involvement of the Third Doctor, the Exxilons and the Subterranean Exxilon rebels, (PROSE: Death to the Daleks) the expedition gave rise to various shifting temporary truces and alliances, even among the humans and Daleks. Most of these were broken off as soon as numerous raids brought down the Great City and restored power to the off-world vessels.

The Daleks had set Exxilon slaves to work mining the parrinium but their supplies were secretly replaced with bags of sand by Sarah Jane Smith and Jill Tarrant. The Dalek saucer departed intent on bombarding Exxilon with plague missiles but the vessel was destroyed by Dan Galloway, who stowed away onboard with an explosive charge. The surviving marines were able to transport the parrinium back to the Outer Worlds to deal with the plague (TV: Death to the Daleks) and the war ended thereafter. (PROSE: The Secret Lives of Monsters)

Aftermath
The war resulted in a tremendous death toll among humanity. Steven Taylor cited the Third Dalek War, as well as its 26th century predecessors, as the interplanetary wars which brought the vast population growth of the Earth Empire to an end during the period and thus concluded the empire's expansionist phase. According to this assessment, it thereby led to many planets originally marked for colonisation, such as Mechanus, remaining uncolonised and the Mechonoids, Earth's servant robots, completely fading from memory. The robots continued to wait for the human colonists who never arrived. (PROSE: The Chase) In 2382, Jyl Stoker claimed the Mechnonoids were outdated long before then, (PROSE: Fear of the Dark) but another historical account, about the history of the universe, supported Steven's assessment. (PROSE: The Whoniverse) The Mechonoids themselves eventually developed sentience and became enemies of the Daleks. (COMIC: Eve of War, The World That Waits, PROSE: Birth of a Legend et al.) while Mechanus grew into the heart of the Mechonoid Empire. (WC: Planet of the Mechanoids)

Four Dalek survivors remained on Exxilon. (TV: Death to the Daleks) Some survivors were driven mad by their encounter with the Doctor and ended up in intensive care in the Dalek Asylum. (TV: Asylum of the Daleks)

The events of the war were recorded in A History of the Dalek Wars, a book by Hagan Garsonomous. (PROSE: The Secret Lives of Monsters)

By the 30th century, the native time period of Chris Cwej, (PROSE: Head Games) a highly fictionalised account of the Third Dalek War served as the premise for the holovid series EarthDoom XV. Cwej watched it as a child. Despite its low production values, it terrified him. (PROSE: Sky Pirates!)

The book about universal history seemed attributed the plague and the Exxilon expedition as stemming from to the Dalek conflict that was sparked in the year 4000 by the attempt to develop the Time Destructor. However, its wider account seemed to offer a more general and not-necessarily linear account of the Dalek Wars as a whole. (PROSE: The Whoniverse)

Retroactive recognition
The Third Dalek War was first mentioned by Steven Taylor in John Peel's 1989 novelisation of The Chase. In the televised serial from 1965, Steven originally made a more general reference to "interplanetary wars", but these were replaced by more specific examples in the novelisation.

Future stories in the Virgin New Adventures, Lucifer Rising and Sky Pirates!, contain further brief references to the Third Dalek War. However, more stories tend to concern themselves with the Second Dalek War, such as Love and War, Return of the Living Dad, Deceit, Abslom Daak... Dalek Killer and Prisoner of the Daleks.

Peel's novelisation mentions the Third Dalek War as the conflict which took place after the Human-Draconian War, with no mention of the Second Dalek War. It is possible Peel intended for the Third Dalek War to refer to the conflict sparked in Frontier in Space, but the New Adventures later established this conflict as the Second Dalek War (which nevertheless became subject to a few retcons itself). Deceit contains a reference to "another Dalek War" breaking out in the aftermath of the Second, helping to maintain a sense of consistency.

Published in 1991, The Terrestrial Index claimed that the Third Dalek War was indeed the conflict which followed Frontier in Space, the Second Dalek War having began in the Exxilon Incident, dated to the 25th century. The Third Dalek War was eventually followed by the Dalek-Movellan War, the Fourth Dalek War and the Dalek Civil War, the Final End.

Eventually, The Secret Lives of Monsters established the events depicted in Death to the Daleks - the Exxilon Gambit - as taking place at the end of the Third Dalek War. Death to the Daleks therefore retroactively became the first story confirmed to depict a part of the war.

AHistory, A History of the Universe and The Dalek Handbook all date the events of Death to the Daleks in or around the year 2600. They seem to treat the Exxilon Gambit as part of the aftermath of the Dalek Wars rather than as part of the Third Dalek War itself, but The Secret Lives of Monsters supersedes them as a valid source.

If the Third Dalek War does take place during the 27th century, it is possible that it merged with, overlapped or is otherwise related to the 27th century Dalek invasion depicted in The Dalek World. The latter conflict may have been subject to tampering with time travel.

The Whoniverse apparently places the plague and the Exxilon expedition, as well as other Dalek conflicts, as taking place in the decades following the events of The Daleks' Master Plan, meaning it would be part of the Great War. Most inconsistencies may be excused by the fact that The Whoniverse is presented as an in-universe work of history which could easily be subject to some of its own inaccuracies.

Battles in Time Magazine
A Dalek Wars source from Doctor Who: Battles in Time #55 gives an account of the final battle of the Third Dalek War, although it is fought by the Second Great and Bountiful Human Empire rather than the Earth Empire, which would mean it wouldn't fit into the same time period as the Human-Draconian War, as established by The Chase above.

According to the account, after being all but defeated by the humans, the Supreme Dalek sent the remaining forces to attack the planet JT227, a planet rich with the element Promethium 62 which the humans were using to break Dalek forcefields. However, the humans' Spindroids mining the element fought back and proved resistant to Dalek fire and the Daleks were pushed back from human space.