Actually, the films were originally made for television, if I remember correctly only the first one, the one you call the dragon tattoo, was intended to be shown in cinemas and stuff. But then it turned out to be hugely popular, so they quickly decided to throw all of them out there. They premiered only a few months after eachother, and the dvd's came really quickly aswell.
Personally, I hate them - the first one was ok, but I turned off the second one after 20 minutes
. I am never going to blame hollywood for showing pointless violence again, because the swedes are now the ones who have perfected that art...