Speaking at the Valve Index part, Newell was quoted as saying, “In our history at Valve we have had a lot of significant milestones. Index is another one of those critical milestones for our company… it represents a significant breakthrough in the field.”
He added, “Milestones aren’t really the end of anything. They’re really the beginning. So Half-Life led to Half-Life 2, Source led to Source 2, the experiments that we did with Team Fortress 2 were what enabled us to build Dota. Artifact is the reason we’re able to do Underlords. So, maybe someday the number 2 will lead us to that shiny integer glowing on a mountain someplace… we’ll just have to see.”
While this is by no means a confirmation that Half-Life 3 is in development, Newell’s comments certainly suggest that the company is open to the idea of a Half-Life 3, but whether or not they’ll go through with it remains to be seen.