I know the fate of heroes from D1:
The warrior hero from first Diablo became the Dark Wanderer and Diablo himself in Diablo 2; the rogue became Blood Raven and the the Sorcerer became the Summoner impersonating Horazon, and they all were killed by the heroes of Diablo 2.
But the story doesn't tell what happened to the heroes of D2.
Do heroes from D2 appear in D3? If not, are they mentioned in D3?
Another slight edit:
The official Diablo 3 Game Guide is now live, and includes an introduction which sets the stage for new players entering Diablo 3 without the background of the previous games. I believes backs up the updated content in my first edit.
Now, a star has fallen from the heavens—a grim omen that heralds the End of Days. Sanctuary’s frightened inhabitants are turning to abandoned legends and prophecies for answers. They have no hope against a full onslaught from the Burning Hells. The heroes of the past are gone or dead.
A new champion must rise…
Edit:
So I've done some additional research, and I believe I have found the most credible Blizzard sourced information on the subject. At this link you can see a video in which a Blizzard panel member from Blizzcon 2011 explains that generally, you don't know what happened to the heroes from Diablo II. They specifically state that all that is known are hints and vague references, deliberately. They do go on to confirm a special case where you run into the apprentice of the Necromancer from Diablo II during a chance encounter in Act II. This thread on Blizznet also discussed some of the known lore related to the Necromancer's apprentice.
You come across him and his three skeleton minions, and he asks you to help him fight off some restless ghosts that the Cultists disturbed. In dialogue, he mentions something to the effect of his master being one of the heroes who helped defeat Diablo 20 years ago.
It is also confirmed in that same thread that the male Barbarian from Diablo III is not the same Barbarian from Diablo II, even though this is often speculated - since that was an original plan by Blizzard that was scrapped due to gender differences, and difficulty in plot integration.
Beyond those two knowns there is some speculation about the Sorceress from Diablo II being tied to a Blizzard short story that provides the backstory for the female Wizard but that is outside the game, and speculative.
There is also mention that by Brother Malachi, the healer in Act 1 that "Zakurum has fallen to disgrace," which bodes ill for the Paladin from Diablo II, if he is still alive.
From what I have researched, the Druid, Amazon, and Assassin are not mentioned at all.
So, I'll leave my original answer below with the grain of salt that there does not seem to be a solid reference. It is a widely distributed version of history at this point that Blizzard neither confirms nor denies, to my knowledge.
According to the wiki and numerous other sources Diablo 3 takes place 20 years after the events of Diablo 2, so the previous adventurers may be too old to swing morning stars against the minions of Hell at this point. The wiki also specifically mentions that Tyrial sent the previous heroes off to live in safety as a reward for their deeds in the last game. So my old Barb is likely bar tending at the Masculine Grunt Inn, telling patrons "This is yours... For you... A gift for you."
The game takes place in Sanctuary, the dark fantasy world of the Diablo series. Sanctuary was saved twenty years prior by unnamed heroes in Diablo II. Having survived that onslaught, Tyrael rewarded the heroes by sending them to safety. It is up to a new generation of heroes to face the forces of evil threatening the world.
With Reaper of Souls now out, you can meet the Sorceress from Diablo 2 when playing through as the Wizard - her name is Isandra and she is the Wizard's former mentor.
An aprrentice of the The Necromancer from D2 makes an appearance in Act II in the desert.
There is an event where you can kill these demons he has trapped.
One of the regulars in the Slaughtered Calf Inn in Act I mentions missing a (probably just recently deceased) bard who used to sing "Bartuc and Horazon", moving people to tears and making them want to change their life.
To make things clear up...
After Aidan the Warrior, Moreina the Rogue, and Jazreth the Sorcerer defeated Diablo in the original game, they went to their own paths as independent travellers. Aidan, the Dark Wanderer was corrupted by Diablo in his soulstone went to free his brothers. While Moreina the Rogue went back to the Sisters of the Sightless Eye, Akara, Kashya, and Charsi felt a negative soul within her and later on corrupted by the demoness Andariel and nicknamed "Blood Raven". Jazreth on the other hand seeked Lut Gholein in curiousity of the wizard Horazon and later on merged with the wizard, now called The Summoner.
As of the new heroes in Diablo II, If you finished the game, you know that the Worldstone is going to destroy the mountain and it's inhabitants. I have a conclusion that only two of the heroes survived the destruction. Xul, the Necromancer, was the only one survived as he has an apprentice in Diablo III, Act II. Isendra, the Sorceress, is confirmed dead in Diablo III: Reaper of Souls expansion where she is the mentor of the current Wizard class. The Barbarian Class in Diablo III has a description saying "survivor" about the northern destruction. Which can be possible that the Barbarian in Diablo II is dead, or alive. Paladin too is unconfirmed if he actually survived but he is indirectly mentioned in Diablo III. The Amazon, Assassin, and Druid have no clues regarding about their whereabouts and can possibly considered dead in the Worldstone Destruction.
This might not be the best explanation in this question forum. But I just wish to help others a few information. Thank you for reading my opinion!
I think I know what happened to the paladins, but this may be wrong. In a book in Diablo 3 it says that the Zakaruum crusaders traveled west, but the paladins traveled east. So this may indicate that if you were a paladin in d2 you left with your order.
