Overall, it depends your:
1. budget
2. the resolution you want to play at
3. the graphic settings
4. what you consider to be acceptable frame rates (FPS)
Here are some benchmarks for The Witcher 2 as reference. The Witcher 3 uses the same game engine as The Witcher; REDengine 3; but I am sure it has been updated since 2011 and has become a bit more demanding.
The MSI GP Series GP60 Leopard-836 linked by the above poster has a GeForce 940m graphics chip and a 1080p screen. That is a little less powerful than the GeForce GTX 660m so if you were to play The Witcher 2 you will get lower FPS compared to that graphics chip. The GeForce 940m would be good enough to play The Witcher 2 at 1366x768 resolution with medium settings and will likely do okay with The Witcher 3 using similar settings. I assume 1600x900 resolution and low settings will provide good performance. Playing at 1080p resolution definitely means you need to play on low settings and you can probably get around 25 FPS to at best 30 FPS.
The GeForce GTX 680 is a desktop graphics card. In terms of performance the closest mobile graphic chip that comes close to it's performance are the very high end Radeon R9 280x, Geforce GTX 860m SLI (dual graphic chips), and GeForce 970m.
Based on what is available at Newegg.com the least expensive laptop with the GeForce 970m is the 17.3" DELL Alienware 17 for $1,400 and it comes with The Witcher 3. The least expensive 15.6" laptop with the Geforce GTX 970 is the MSI GE Series GE62 Apache Pro-055 for $1,538; probably due to extra cooling in a smaller laptop because the GTX 970m produces a lot of heat.
http/www.newegg.com/Product/Product.aspx?Item=N82E16834299296&cm_re=gtx_970m-_-34-299-296-_-Product
http/www.newegg.com/Product/Product.aspx?Item=N82E16834152740&cm_re=gtx_970m-_-34-152-740-_-Product
Sooo..... Basically you need to determine what price point between about $800 to around $1,600 works for you.