Valve sent out a press release saying they will not be showing off any titles (either new or in development) at E3 but also didn't specifically mention if they will also not be going to E3.
Unless Valve is planning a surprise secret announcement like last year, no chance at HL2: Episode 3 or anything else will be shown.
http://www.giantbomb.com/news/son-of...es-at-e3/3212/