Update: Is it really happening? No. No it isn't.
Valve just pushed a graphic for Half Life 3 onto its main site. Or at least a site that looked like Valve's official site.
A site masquerading as Valve's official page pushed a graphic for Half Life 3 onto its main carousel a little while ago, in the middle of the PC Gaming press conference at E3 2015.
However, the site gave itself away when it dumped to Valve's Steam Powered storefront and the actual Valve website.
That was an epic fucking troll. Bravo, you asshole.