Portal and Portal 2 are two of the best games in history. They're arguably just as good as another pair of Valve games, Half-Life and Half-Life 2. Both are also the reason why there's a meme about how Valve doesn't like the number 3. Despite significant fan interest, Valve doesn't seem interested in making Portal 3 or Half-Life 3, but this hasn't stopped the people involved in the making of the said games from trying.
Case in point, Portal and Portal 2 co-writer, Erik Wolpaw, just called out Valve to let him do Portal 3.
In a recent Kiki Talkz podcast, VGC reports that Wolpaw's message to Valve, or specifically, Gabe Newell, is that "We've gotta start Portal 3." Wolpaw added that he isn't "getting any younger" and he doesn't want to wait before he's "too old to work on Portal 3." Wolpaw went as far as to encourage fans to "send mail to Gabe" if they want Valve to make Portal 3. Finally, Wolpaw mentioned that Valve stands to make a lot of money from Portal 3, even if it's not gonna make them "Counter-Strike: GO Money.
Unfortunately, the threequel-averse studio hasn't announced any plans to revisit the Portal games anytime soon. The closest thing that Portal fans have to a new installment that they can look forward to is a port for the Nintendo Switch.