So according to anonymous source Half Life 3 is developed by 10 people

Watch the video:

https://www.youtube.com/watch?v=50lSIaSR3zc

Here's a short summary I found on Neogaf:

HL3:
1. in development but by a small crew of ~10 people currently
2. project status is the script is complete
3. valve management not invested in releasing it anytime soon
4. was in development with larger team in 2009 but devs got moved to other projects which needed attention
5. mass effect 3 backlash influenced them. devs saw how passionate fans turned on the game and didnt want that to happen to the company's banner franchise
6. game culture has changed since 2004 ("sadly, most people are shitheads" - anonymous source at valve)

$$$:
7. cash flow from steam and F2P microtransactions is more than sustaining valve
8. staff moved to projects that bring in the money
9. Dota, CS Go, TF2 earned valve over $400 million in 2014.
10. Conservative estimate for total steam revenue in 2014 at $1.5 billion (1st and 3rd party)
11. Valve's 30% cut of $1.1 billion (3rd party only) combined with their top 3 F2P game earnings = $730 million in 2014.
12. no financial pressure to release HL3 (generous 12 million units full price retail for HL3 equals $720 million).*

Seems like it's in development hell, lets hope it won't turn out to be another Duke Nukem Forever.