Third Doctor's regeneration

The Third Doctor underwent his regeneration into the Fourth Doctor as a result of radiation poisoning upon prolonged exposure to the Great One's highly unstable web of Metebelis crystals. (TV: Planet of the Spiders) This was the third regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

A day to come
The Third Doctor briefly began to regenerate after battling a mind parasite known as the Nurazh. The Nurazh attempted to take control of his mind, but was overwhelmed by both the Third and Fourth Doctor's minds co-existing. The parasite parished, healing the Doctor and reverting the regeneration. Jo Grant witnessed this, and described to the Doctor his next incarnation. (PROSE: The Touch of the Nurazh)

Interference
Due to interference caused by Faction Paradox, the circumstances of the Third Doctor's regeneration were altered, so that instead of regenerating on Metebelis III, he regenerated on Dust, although Mother Mathara believed there would be no substantial difference in the timeline until the Eighth Doctor. (PROSE: Interference - Book Two) The Eighth Doctor eventually rectified this. (PROSE: The Ancestor Cell)

Legacy
Reunited with the Third Doctor in the Death Zone, Sarah Jane Smith reminded him of when he changed into what he knew as "teeth and curls", only for him to explain that it was yet to happen for him. (TV: The Five Doctors)