Based on how the last Portal 2 game ended. Most people assumed that there was going to be a new Portal game in the near future but Valve has never made that announcement. In fact, they have never hinted that a new Portal game could be happening but the fans think that it will happen sooner than we think.
It all started with the mysterious ending where Chell was seen in a field with a companion cube. Some people think that the story has not ended yet but even if that is true, that does not mean that Valve is ready to start working on it again.
We do not know what Valve have against the number 3 but they often stopped at 2 and leave it there. We have the Half-Life 2, Left 4 Dead 2, Team Fortress 2 and those are just some of the examples.
Some people believe that Valve will be making an announcement this year as it has been 5 years since we last heard of the game but we personally do not think that it is going to happen. Do you?