NeoGAF has discovered a filing from Valve registered on September 29th with Europe's trademarks office, the OHIM, for Half-Life 3.
No details are provided and the Community Trade Mark listing simply states that the application is for:
"Computer game software; Electronic game software; Downloadable computer game software via a global computer network and wireless devices; video game software."
Valve is mentioned as the owner of the trademark and made the application in both English and French.
Last week, Valve made a series of announcements starting with SteamOS which is designed to bridge the gap between Valve's platform and the living room; followed by Steam Machines – which will be available from the beginning of 2014 and you can currently sign up to the beta.
Finally, the company announced a controller.
While a trademark application doesn't mean Half-Life 3 is in active development the game's launch would make sense when SteamOS and Steam Machines hit the market.