Cain returns once more in Diablo 3 as a supporting character from doris89592's blog

During the four acts of Diablo 2, Diablo IV Gold rapidly corrupts Prince Aidan and the Prince starts to lose control over the Lord of Terror's soul leading to the release of Hell's demons once more onto Tristram. Cain returns once more in Diablo 3 as a supporting character. 

The player, a different nameless warrior, rescues Cain, and from then on Cain would be the hero's constant companion, offering to identify magical items free of charge and provide advice from his repertoire of ancient knowledge to once again defeat the demon Diablo.

The player character, known as the Nephalem regardless of which Diablo 3 class they choose, is implored by Cain's ward Leah to rescue the scholar from the ruins of Tristram Cathedral which had been struck by a 'fallen star.' 

The three discover that the 'fallen star' is actually a person, who they refer to as 'the Stranger,' who has no memories aside from knowing that he lost his sword.At the end of Act 1 of the polarizing Diablo 3, the witch tortures Cain to force him to fix the sword using his magical abilities, but he refuses, and is tortured further to the point of death.  

The Nephalem recovers the cheap Diablo 4 Gold sword that was shattered into three pieces, but their efforts were quickly thwarted by the witch Maghda who seizes the sword shards and captures Cain.