I HAVE played Witcher 2, but not the third installment yet. With that in mind, I think you should absolutely play the 2nd one - it's soooooo good. It's like a grimdark version of Dragon Age, only with better characters, world, monsters: everything basically.
The only reason I haven't started the third game is down to personal issues; it's 100% the game I would play next if I could find the time. My history with the series is a Youtube recap of the first game, plus an extensive playthrough of the second one on PC. I have not read the novels, yet my love was cemented by that single game I've played through.
TLDR; by all accounts, they've designed the third game so you can jump right in, but when the Witcher 2 holds up as well as it does, why would you skip out on another stellar experience in its own right?