So, according to the last answer, the 3rd announcement will probably be about a gamepad or an input device of sorts. That's a bummer. I know it was very unlikely, but HL3 announcement would have made me so happy. I'm still hopeful though, since it's been confirmed that Source 2 is in the works. Valve usually shows off their new engine with a new HL game.
While HL3 would be an awesome announcement, it doesn't seem highly relevant to SteamOS. I'm sure if anything, they'll release HL3 as a Steam-exclusive game as a launch title.
