Is it worth to get W3 without playing the first 2?
No one played W3 yet, so you wan't get the answear. But, in one of the interviews they said that they will introduce the player into the story at the begging of the game, some kind of a prologue, so I guess it's not really necessary to play previous the witcher series games or even read the books to get familiar with the main protagonist Geralt of Rivia.
But I recommend to try out The Witcher 2, damn good story and nice gameplay. Some of the fans of the Witcher books may not agree about the story as it's not really close to what they were expecting, mainly becouse it lacks the spirit of the witcher known for them from the novels, but fuck it. It is still a damn good game.
First game, was really great as well, but at this point the gameplay mechanic and graphics (if you care about it) may seem a bit outdated. Anyway, I highly recommend to play it.
I'm on chapter 4 in the first game and it only took me 3 years to get there.
Chapter 4 is the best part of this game