Half-Life 3 is undoubtedly the most anticipated sequel to any game ever released. The first two games have set the bar of the quality so high that it will be hard to achieve the same level of intrigue as before.
However, if you want to know how the development of this incredibly popular series began, then be sure to check the developer commentary of Half-Life 2: Episodes 1 & 2.
Unfortunately, the development team didn't record commentary for the first game, but there is more than enough material to study from the two episodes of the sequel. Commentary includes anecdotes and behind the scenes stories from team members such as Gabe Newell, Greg Coomer, Jay Stelly, and many more.