I am have 3 theories:
1. Valve is only working on Steam and his popular games so Gabe is forgot about HL3 and other games with 3.
2. When is Gabe was child, he was haved 3 dollars. One ice cream who is wanted to buy is was cost 1 dollar. After is left 2 dollars so he is begin to like that number.
3. Valve is obviusly now working on all games with number three, so Valve is publish it on one pack called 'Three Pack'. That project is too big so that pack will be published for, maybe, 3-4 years.