The Witcher series developer CD Projekt RED has started posting classified ads for positions with the company on its site, stating the developer is "staffing up for its next project".
While this could be for either the Xbox 360 version of The Witcher 2, or making DLC for the PC version, there's evidence that the company is beginning work on The Witcher 3.
According to "Consider this example, for comparison, films like Pirates of the Caribbean, where the first (has a) self-contained story, the later is unchangeable, but then you think, it should be a trilogy. So people have a problem then. The end of The Witcher 2 is the beginning of much larger events, while The Witcher (had) rather a cliffhanger ending.", Jan Bartkowicz stated,