Strange question: I'm playing the Dominions 3 demo right now, and I'm already planning to buy the full game when I put some money on my credit card.
Do I miss something if I just play Dominions 3 and not Dominions 1 and 2 in regards of the singleplayer part of the game? Like a campaign or something like that?
There's history, though. You get to see it in little bits and pieces in nation/unit descriptions. Many of the nations have shared history, much of it revolving around the Fall of Ermor. It really is a fascinating, and fairly coherent world.