You could probably go into Disgaea 3 without any prior knowledge of one or two, but it usually helps a little, i mean in 2 you had the main characters from 1(Laharl and Etna primarily) appearing in the main storyline, but it doesn't mean you have to know who they are as its explained to enough detail for 2. Its just always quite nice to know a bit more though, hah. As for 3, i'm not entirely sure what to expect in it, i know there will be DLC in terms of extra character fights and so on, but i'm not sure too much about the storyline. My brother says in terms of chapters, there is less but more levels per instead and feels longer, plus the new additions to the battle system are quite nice. Only downside apparently is that there is less Prinnies.
And yeah, Disgaea is not a game you go into for graphics, your characters are fine anyway, but its the maps that people would probably have gripes at with, but why i wouldn't know, thats nothing to even bother with, its gameplay and story you want a game for, or at least it used to be from what i remember =/