No, Valve did not announced Half-Life 3, until NOW! We hop this will change next days, but they annouced some other things at Game Developers Conference. An few days ago we talked about the anounncement of their VR Vive.
First, they announced Steam Link which will enable game streaming to Steam Machines, PC, Mac, and Linux PC from another PC or Steam Machine within the same network. This will support 1080p/60Hz gaming. Steam Link will be available this November for $49.99. For $49.99 more users can get a Steam Controller. Those interested in Steam Link can follow it on Steam for updates.
In addition, Valve revealed the Source 2 engine. This engine has been made with a focus on “increasing creator productivity.” Source 2 will be free to content developers. Source 2 allowing “gamers themselves to participate in the creation and development of their favorite games.”