Portal 3 vs Half Life 3: Portal Will Win The Battle
If it were up to Gabe Newell, Portal 3 might win over Half Life 3 just because Newell is more involve with Half Life 3 then he was with Portal 3.
During an AMA session with Gabe Newell, a fan asked if Gabe preferred the Portal or the Half-Life franchise. He answered that because he was more involve in Half-Life and can see all the mistake he made with the game, it is earlier for him to like Portal more.
Of course, that is far from any confirmation and it does not mean that a Portal 3 is on the way. In the same AMA session, Gabe Newell also said that in Valve, there needs to be somebody that actually wants to create the game because they stat considering it.
Maybe nobody in Valve is interested in any of these games anymore and we should probably forget about it.