It seems Valve DO know what the number 3 is since they have recently filed a trademark for Portal 3. This trademark was filed on the 2nd of October 2013 and was eclipsed by the news that Half-Life 3 had also been trademarked (and then confirmed to be fake!). 

We want to believe this trademark but we have been burnt once before. At least with the HL3 trademark we at least knew they were working on the game, but we have heard no news about Valve turning the Portal series into a trilogy…

What do you think? Is a Portal 3 coming? Or we just being diddled again?