ValveTime are reporting that forum member "tabernumse" has posted the e-mail response he received from Gabe Newell in reference to their plans for E3, shown below:
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, ...)
On one hand, I'm upset. I really wanted a Half-Life 3 announcement, but in typical Valve style, they'll wait. I'll let them do this, I'll let it slide. But come E3 2013, they'd better not only announce Half-Life 3, but release it close to E3. Release it on their Steam Box, release it on PC, give it to us. Dump out Left 4 Dead 3 and Team Fortress 3 at the same time, wouldn't that be nice.