Of course given how obscure this is, “hl3” could very well refer to just about anything. However we are talking about Valve, not to mention gamers have been clamoring for Half-Life 3 so there is a good chance that “hl3” could indeed be a reference to Half-Life 3. For those interested in taking a look at the code themselves, it can be found in %STEAM_HOME%\SteamApps\common\dota 2 beta\dota_ugc\game\bin\win32\tools\model_editor.dll.
At this point in time it would really be impossible to tell if and when Valve will announce the game, let alone release it. It seems unlikely that the company would be ignoring the franchise, especially when you consider how many gamers out there have played and loved the games, and let’s not forget how some of today’s more popular titles such as Counter-Strike got its start as a Half-Life mod.
In any case try not to get your hopes up too high just yet, but hopefully (fingers crossed) this reference is a sign of things to come. Anyone else care to venture a guess as to what else the code could be referring to?