Pfft, I want a whole new engine. Bring on HF3!
Dude, Source is a fking outdated engine, funny how the Crytek engine was released only at 2007, yet still people struggle with it... (not me though =D).
Anyways, I doubt HL2:EP3 will be on the Source Engine, Source was awesome at 2005, but now it is no match for current graphics. HL2:EP3 is an extremely anticipated game, so I doubt that they were going to use the same Source engine, that's just boring. Maybe it took them so god damn long to start developing the game, because perhaps they were developing a new engine??