Valve has just confirmed via ValveTime that the studio has nothing to show at the upcoming E3 convention, which is set to take place in early June.
We are not announcing anything at E3. Really. We are not announcing Half-Life 3 or Portal 3 or Left 4 Dead 3. We are going to be showing stuff everyone already knows about (CS:Go, Dota 2, 10' UI, …)
A great deal of speculation has been made regarding Half-Life 3, especially over the recent days.
It looks like we'll just have to keep on keeping on until Valve makes a proper announcement about the game at some point. We'll just have to be satisfied with the Portal 2 DLC, CS:GO and DOTA 2. With any luck, Valve will pull through with something by this time next year.