I'm jealous of anyone just starting this game.... be prepared for insane length.
I didn't play 1 or 2 but I did read the entire series of books that were written in the 90s before playing this. Really good. It helped me appreciate the game more in some ways, in other ways it irritated and disappointed me as I had now become a fan and therefore wasn't keen on every aspect of their adaptation. The games are literally fanfiction, they are continuations of the plot in the novels but without author involvement. The stories and writing, while great for a game, just aren't very good compared to the books. I think they got some of the characters wrong, but none more than Geralt. He's much cooler and more interesting in the books. In TW3 he's a real wet rag, I hate how flat the actor's delivery is.
Still, amazing game and really raises the bar for RPGs IMO. I played Dragon Age Inquisition righ before this and Fallout 4 right after. DA:I was an embarrassment and FO4 was a bit of a let down. Both failed badly at what TW3 does so well.