Most people have already given up on Half Life 3. Despite the ending, it is clear that Valve does not have any intention to revisit the story as they shift their focus on multiplayer online games like Dota 2.
The same thing can almost be said about the Portal 3 game. So far, Valve has not said anything about there being a Portal 3 game in the making. However, if Valve does decide that they want to revisit any one of these old game, it looks like Portal 3 might have a bigger chance of getting picked up just because Gabe Newell seems to prefer it over Half-Life.
During his AMA session on Reddit, Newell was asked if he preferred Half Life or Portal and he answered Portal. His explanation was that since he had a huge role to play in the development for Half-Life, it is easy for him to see all the flaws and the mistake that he made with the game and that is why he finds it easier to like the Portal game.
He might like Portal more but we don’t think you should get your hopes up yet.