Who knows? Valve might, but they've never announced it officially so on that basis I think it's unreasonable to moan that it's taking too long. It's perfectly fine to criticise them for taking so long to release HL2 Episodes when their entire purpose was to speed development times. But HL3 hasn't been announced and as such is not a valid cause for complaint.
To add to the discussion, with Valve's unique internal structure (or lack thereof) being what it is, if HL3 *is* in development (let us assume that it is) then it could be being worked on by a single developer, a small team, a huge team or the entire company for all we know.
Add to this Valve's preference (and my own) for games to remain unreleased until they're ready, if HL3 is in development it's better that it comes when it comes and is incredible, than to ship sooner with less content, polish or quality. For one thing, after the first two games and their legendary status, to make an equally good or better game capable of meeting (or exceeding) player expectations is a monumental and unenviable challenge.
Lastly, I personally prefer fewer, quality, well-supported games than the EA/Activision style of re-releasing the same, identical game annually with and incremented number and new maps, double the cost of excluded content to be nickel-and-dimed back in over 11 months and the same boring, bland and uninspired gameplay that I don't spend a penny on anyway. A quality addition to a well-loved franchise has to strike a balance between "more of the same", new and interesting and expanding on what's come before. Better to have fewer, better games than more rushed identical ones. Still HL3 has been an awfully long time in coming. I think 3 years is about right to aim for, 4 to really nail it. More than that is missed opportunity.
Imagine how many good games could have/would have been great games if they'd been released when ready, and not before? Alpha Protocol, KotOR2 and VTM: Bloodlines for starters. Better to wait, no need to announce things early, raise expectations and hype then fail to deliver. Quality beats quantity any day.