So maybe it's not the precise Valve game with a 3 in it that we're waiting for. Still, any news of a 3 coming out of Valve within our lifetimes is probably good news. In this case, it's a hint of zombie shooter Left 4 Dead 3, revealed in an image of a Valve employee's folder directory.
ValveTime, a Valve fansite, posted a screenshot taken from a tutorial by Valve artist Tristan Reidford for Destinations Workshop Tools. In the screenshot, a folder named left4dead3 is visible. You can see it the image below, at the top left of the folders.
I would personally say it's suspiciously conspicuous at the top left of the folders, almost as if the directory had been purposely scrolled down just enough to see it. But who knows?