Having not played DIII yet, I really can't comment, however if you haven't played Heroes of Might and Magic 2 & 3, or Disciples 2 yet, I'd consider starting with those first (though they can be brutally difficult at times), unless the graphics are a complete turn off for you. In which case, I'd move to Heroes V: Tribes of the East (self-contained expansion - best of the Heroes V games), Kings Bounty, or possibly even Heroes VI before Disciples 3, though the sale price of Disciples 3 may be a factor to push that ahead.
Graphically I think they'd fall:
Heroes VI > Disciples 3 > Kings Bounty > Heroes V > Disciples 2 = HoM&M III/II.
Gameplay it'd be more:
Heroes 3 > Heroes 2 = Disciples 2 = Kings Bounty > Heroes V > Disciples 3