No way...
EDIT yeh, confirmed rubbish.
http://www.eurogamer.net/articles/2012-08-14-gamescom-claims-half-life-3-and-dragon-age-3-will-be-at-show
Update: Dragon Age 3 and Half-Life 3 appearing on the Gamescom-compiled list of games at the show was "a mistake", the organisers have told Eurogamer.