Remember that Half Life 3 leak that appeared on Steam earlier this year? Unfortunately, it was a prank pulled by an employee. One Half Life fan has performed an extensive research on the matter and realized that the Half Life 3 logos that surfaced on Steam are both fan-made.
There are two logos that surfaced. The first logo is revealed to look identical to the ones promoted at Reddit about a year ago. As for the second logo, it was sourced from Deviantart. This basically proves that both logos were not developed by Valve at all.
Hence, there is no Half Life 3 in development and the leak is nothing more than a prank. The only way for it to be real is if Half Life 3’s logos were created by Valve and they look identical to the fan-made versions by coincidence.