Heroes from previous games
This list contains all heroes from "Heroes of Might and Magic: A Strategic Quest", "Heroes of Might and Magic II: The Succession Wars", and its expansion, "The Price of Loyalty", who reappeared in "Heroes of Might and Magic III".
Alamar the Warlock
Crag Hack the Barbarian
Gem the Druid (appears as a Sorceress in Heroes 1 and 2)
Halon the Wizard
Lord Haart the Knight
Sandro the Necromancer (appears as a Warlock in Heroes 1, and a Necromancer in Heroes 2)-
Yog the Barbarian 
Luna the Elementalist 
(appears as a Sorceress in Heroes 1 and 2)-
Roland the Knight 
Agar the Artificer 
(appears as a Warlock in Heroes 1 and 2)
Astra the Navigator (appears as a Sorceress in Heroes 1 and 2)-
Elderian the Artificer (appears as a Wizard in Heroes 2 PoL) 
Ranloo the Death Knight (appears as a Necromancer in Heroes 2)-
Wrathmont the Artificer (appears as a Warlock in Heroes 1 and 2)
Trivia
"The Price of Loyalty" features a wizard named Solmyr, however it is a different character from Solmyr, as he is a human, not a genie. Also PoL's Solmyr is allegedly non-canon as he only appears in a series of scenarios instead of in a campaign.
Kilburn the Knight, who appeared in Heroes 1 and Heroes 2, was featured in the beta version of Heroes 3. However, he was cut from the final game.