NeoGAF member shinobi602 has spotted a listing for Half-Life 2: Episode 3 in an online GameStop catalog only viewable to PowerUp Rewards members. The title has no release date, not art, and is listed only for PC.
I really don't know what to make of this. Valve has said they have no product announcements planned for E3 and while those plans can definitely change between now and the show (they may already, just to acknowledge the possibility), it's extremely doubtful Gordon Freeman's next adventure will be another Half-Life 2 add-on.
I expect Valve to confirm shenanigans should this gain traction, but I'll check my own sources in the mean time.