Searching for the registry number at the OHIM will find it doesn't exist anymore, but all of this doesn't explain the numerous Half-Life 3 references, images and logos that appeared in Valve's internal groups, as seen below.
It seems like either an external party committed a hoax, or Valve prematurely registered Half-Life 3 while it's still in development.
Adding to the intrigue, an OHIM registry for Portal 3 has appeared, and this entry was vetted by OHIM itself, meaning it's confirmed.