At this point, most people believe that Valve has already given up on Portal 3 just like Half Life 3 but some people still believe that we might hear more about the third installment before 2016 ends.
In fact, some of them even believe that Valve will release the Portal 3 at the end of this year. As much as we would like it to be true, there has never been any indication that Valve is working on the third installment right now. In fact, if they were to release the game this year, we would have probably heard the announcement already but no such announcement was made.
The reason why so many people believe that a third installment is going to happen was because of the mysterious ending in the last game where Chell, was placed in a field with a companion cube.
Maybe Valve wanted to leave it open so that they can pick it up if they wanted to but that does not mean they will. Valve seems to be focusing more on their multiplayer games right now.