Nah, just a whisper in the wind at this point.
Valve (or at least Gabe?) thinks classic gameplay doesn't sell anymore or for some reason their open ended development teams have no mass interest in simply rolling their chairs over to a HL3 section, even though classic style games made a heavy return in recent years. HL3 maybe could sell like hot cakes if it kept to the original formula.
"The only reason we'd go back and do like a super classic kind of product is if a whole bunch of people just internally at Valve said they wanted to do it and had a reasonable explanation for why [they did]." - Mr. Gabe