Do I need to play previous Witchers before Witcher3
Solution 1:
If you never played any Witcher game before you might not understand some parts of the Witcher 3.
Let me explain.
You will not experience that little smile or frown when you meet a character from a previous game (Good or Bad ;) ). Some parts will just fly above you and you won't understand much.
"Who is this?"/"Why is he treating me like this?", you won't notice the little easter eggs found across The Witcher 3 for references to previous games.
I mean, you can play it and still enjoy it. But you will have a far better experience if you play the previous ones.
The Witcher 2: Assassins of Kings I highly recommend, great story and you make some big decisions.
The Witcher is a bit old now and the controls can get a little out of control so I would advise you to check a synopsis online.
You can read a synopsis on both games. I've done it myself. There are some good ones and bad ones. But there's always differences between playing and reading/watching a synopsis.
If you're interested I found one in Reddit, a synopsis Chapter by Chapter HERE. Now it's up to you to decide I definitely LOVED the Witcher 2 and making those decisions I believe has more impact than imagining them. Not only from the Main Quests but also for the beautiful Side-Quests.
If you're trully interested and don't have time for the others. Play it. But then if you like it please support CD Projekt RED and try at least Witcher 2 and then carry your decisions to 3.... You'll find it way more appealing and immersive.
Solution 2:
I can definitely say that you will enjoy the story a little better after having played The Witcher and The Witcher 2. However, this is only minor and does not impact your experience as a whole. Therefore, I can also definitely say that you do not need to play the other games.
If you enjoy the story and lore, and want to know about the events leading up to The Witcher 3, I'd recommend either playing the previous games, reading up in a lore primer, or reading the books.
Here's a link to a good, concise story primer that focuses on the essentials: http://www.pcgamer.com/the-witcher-3-story-primer-catch-up-on-the-essentials/