Half Life 3's a pretty cool guy, eh swings crowbarb and doesn't afraid of anything
Valve really loves ARGs ever since Portal 2, Half Life 3 will probably have the biggest ARG Valve has ever come up with
If there will be one, it should have multiple paths that continue and save progress throughout Half Life 2 and the episodes.
A few paths would be fake outs, but the correct path would reveal some things. Of course, the correct path won't flash images of Half Life 3 in your face.
Basically, the Portal 2 ARG except longer and more involved.