Not Portal 3!, cries Valve.
This is not a sequel to Portal, however
it is in the expanded universe of those games.
The demo features a short playthrough in Aperture Science, the facility that both Portal games are set in.
The description states that you will
play as an entry-level nobody on their first day at work.
Aperture Desk Job is available for free on Steam for anyone to play, though a controller is required as keyboard and mouse is not supported. (It is made for a console after all.)