The fact is that Valve never really did hint that they were going to work on a third installment for the Portal game. However, most fans seem to think that it is going to happen, in fact, some fans think that we might even get an announcement real soon.
One of the reasons why the fans think that a third installment was always part of the plan was because of the ending in the second game where we see Chell in a field with a companion cube. Their fans are convinced that it means something but maybe Valve just put it there to mess with us.
There were also talks about Valve thinking about making a Portal and Half-Life game but let’s not go there now.
While we are hoping that Valve would actually start working on Portal 3, we also know that Valve is more interesting in making multiplayer online games right now and Portal is definitely not that sort of games.