Half-Life 3 and Left 4 Dead 3 have been uncovered in internal mailing lists. There was an issue found in Jira, a project management and bug tracking software, so when Valve attempted to resolve the matter the public had an opportunity to access internal information which included these lists.
It seems that some of the games have more groups than other suggesting games in more active development. Left 4 Dead 3 and Source 2 have more mailing groups than Half-Life 3, unfortunately this could mean that while Half-Life 3 could be coming, it doesn’t seem like that will be anytime soon. Half-Life 3 has been on many Valve fan’s mind for a long time and I feel it’s time that Valve delivered. If it will be a while before HL3 at least we have L4D 3 coming, maybe.