Half-Life 3 is not late. Look at the difference in time between HL1 to HL2.
HL1: late 1998
HL2: late 2004 (6 years later)
HL2-1: mid 2006 (2 years later)
HL2-2: late 2007 (1 year later)
Today: mid 2014 (6.5 years later)
you could argue that from this point on, half-life 3 will be getting later and later.
assuming you're looking from half life 2 episode 2 and on.