" It's called HL2: Episode Three. Stop saying Half-Life 3. It's not gonna be L4D3 either. "
Announcing Half-Life 2: anything in 2010 (or the far future) is completely insane from a marketing standpoint. It will be called Half-Life 3 (when it finally happens). "
That answer sounds very stupid. Half-Life 3 needs to be a drastic jump into a new time period and world, not just an easy sequel that finishes the HL2 saga. Don't apply other developer's logic to Valve, it never works.
It will still be called Half-Life 2: Episode Three. They've gone this far, they can't change it now without having to do some really weird explanations for new players.