Despite the ad which briefly appeared on the Steam front page announcing that Witcher 3 is coming to SteamOS, we now received an official answer that it is not confirmed.
In the interview on Gamescom 2014 which consisted of questions collected from the Witcher community on CDPR forums, Environment Art & Level Design Producer Michał Stec of CD Projekt Red was asked whether Witcher 3 is coming to Linux and the answer was not very encouraging:
QuoteQ: Will Witcher 3 be coming to Linux?
A: It’s too early to say, we’re focusing on releasing it on PC, Xbox One and PS 4, same quality, same experience to all the players. We’ll see after that, Linux, Mac I guess it might be planned but I do not know about it yet.
Source (at 06:29 in the interview):
Unfortunately no reason was given for the SteamOS ad on Steam that caused a lot of confusion, and no details about whether any work on supporting OpenGL in REDengine has started so far.
The Witcher 3 is a highly anticipated action RPG developed by CD Projekt Red and scheduled to be released in February 2015. While the question about Linux support was raised many times, CD Projekt red never gave a definitive answer about their plans before, until the confusing SteamOS ad appeared on the front Steam page. It was removed within a day and many were left wondering whether it was legitimate or not. Neither Valve nor CD Projekt Red officially commented on that event, but it is clear that Linux users should not expect the Linux version at release time if they are to make any pre-orders now.
If you want to show your interest in the Linux version of the Witcher 3 you can add your vote to this entry in the GOG wishlist.