Valve has the luxury that they don't have to release games to stay afloat; Steam rakes in more than enough money for Valve to make a hefty profit, so they can just release whatever they want whenever they want to. If I had to guess, Half-Life 3 has been in "development" since HL2: Episode 2 was released. Maybe not as actively as they might have been years ago, but they probably have people still thinking about it. When or if some designer comes up with an idea that's just too good to pass up, then they'll make the game. I imagine Portal 3 is probably in the same boat.