It is nearly the end of 2016, and we haven't heard anything about "Portal 3" from Valve. It would be possible though that the action puzzler game might surface through an announcement at least by next year.
Earlier reports did mention that the leaks of a third game confirmed the possibility of having a third game in the franchise. With a leak accidentally shown by Tristan Reidford, fans of the Valve community saw folders of "Left 4 Dead 3" and "Portal 3" on sides, albeit too briefly.
The screenshot was taken during his tutorial for the Destinations Workshop. However, after the rumors of a 'leak' spread like wildfire on the internet, the screenshot was subsequently removed. Valve has also neither confirmed nor released any statements regarding the leak.
Digital Spy did mention earlier that Valve already filed for a trademark for "Portal 3" back in 2013 with the Trademarks and Design Registration Office of the European Union. This was already confirmed by the actual office itself, which kept fans wide-eyed and hopeful for a third game.
In relation to that, the 2016 trailer of HTC Vive Pre, the VR project that Valve collaborated with, showed a woman wearing similar outfit as Chell in the previous "Portal" games. This could mean a possible VR compatibility for "Portal 3."
Rumors already circulated that the Valve Corporation has a knack for stopping at the second game, never producing a third sequel. That is why we only have "Half-Life 2," "Portal 2," "Team Fortress 2" and "Left 4 Dead 2." Fans could only guess that one of the big names in the video games industry, known for its subtle and elusive nature, would be working really hard to deliver the best kind of revolutionary experiences with its potential third sequels.
