New information has surfaced on SteamDB that seems to indicate that Half Life 3 may actually be in the works. Thanks to a vulnerability in the Steam help site, SteamDB was able to obtain the names that correspond to steam app IDs that they were previously unable to see after Valve removed package names from the subscription ID list. Among the IDs they were able to find names for were packages for Half Life 3, Half Life 3 for Beta Testing, and Half Life 3 Developer Comp, along with packages for Dark Souls III, Rise of the Tomb Raider, and two Final Fantasy titles.
According to the SteamDB, it is entirely possible that these are forged entries from individuals trying to launch a Cross-Site Scripting (XSS) attack on Steam, even though in the past many of these leaks have ended up becoming a reality. SteamDB later updated their statement to include a report that it appears there were indeed entries from Valve.
Half Life 3 Confirmed? Maybe, maybe not, but one can only hope.
Valve has yet to comment.