Dejecting, but it is so. Head on over to Kotaku to read up on the matter:

According to Valve's marketing director Doug Lombardi Half-Life 2: Episode 3 won't be making an appearance at E3 this year, with the listing chalked up to a "misprint (or something)." We would assume that if Valve is showcasing anything at E3 this year, it will be Left 4 Dead which is planned for a November 4 release.