According to Codies, it packs "more than double the track content" of the previous game, Dirt 2, including "the most amount of Rally content to feature in the series to date, accounting for 60% of its extensive career mode." In short, if you like driving cars, this is a game with lots of car-driving. There's no word yet on whether there'll be a demo or not, though both previous Dirt games received one.
Dirt 3--or DiRT 3, as the cool kids stylise it--will hit PC, Xbox 360 and PlayStation 3. The PC edition is a Games for Windows Live title, a fact which will surely be received warmly by some and cursed by others.
I loved the second one. I will pick this one up.