Witcher 2 is a good introduction to the state of the world in 3. You can read the books (or watch the whole series after it concludes), play 2 and understand the geopolitics of 3. It is also short, has a good but nearly-linear story (there are choices that significantly change your playthrough and outcomes, but when you play a second time taking the other road at every fork you've seen it all) and can be had for cheap. Oh, and it has a really touching story arc with the best love scene I've ever seen in any game.
Witcher 1 was a good game for its time, but unless you consider yourself a serious fan of the universe, I would recommend skipping. It's the only one in which Sapkowski was involved, it has some very good dialogues in random places and it's the only one which gives you a very good feel for what a "normal" Witcher's life is. But the voice acting is only good in Polish, up to the point it's recommended everyone use Polish audio with subtitles if they don't speak the language. And the plot is quite disconnected from the books -> Witcher 2 -> 3 line (I'm really bummed we never got closure on the plot of 1).
Witcher 1 was a good game for its time, but unless you consider yourself a serious fan of the universe, I would recommend skipping. It's the only one in which Sapkowski was involved, it has some very good dialogues in random places and it's the only one which gives you a very good feel for what a "normal" Witcher's life is. But the voice acting is only good in Polish, up to the point it's recommended everyone use Polish audio with subtitles if they don't speak the language. And the plot is quite disconnected from the books -> Witcher 2 -> 3 line (I'm really bummed we never got closure on the plot of 1).