As you know, we're dead against April Fools jokes which is why it is only right that we debunk one. It's just not fair to be tricked in such a way.
There is a fake URL being passed around various news sites which, when visited, looks like the Steam Store home page containing a pant-wettingly exciting image of a Half-Life 3 logo and the words "Now Available".
Sadly, however, clicking on this image takes you to this one:
It seems to be widely believed at various sources that this is a real attempt by Valve at an April Fools' prank. Unfortunately though it is clear when you look at the URL: http://store.steamppowered.com/ that it contains an extra "P".
The moral of the story? Don't be fooled. You fools.