I remember reading something coming out of Gabe's big mouth along the lines of:
"We've built a great infrastructure with HL2 and we still have a lot of ideas and written material we want to polish and turn into gameplay, so it would be a pity to have to wait for HL3 before we make that content available."
And thus the 3 episodes were born, because I think it was planned since the start as a trilogy of episodes.
During the hiatus between HL1 and the conception of HL2, many people asked why Valve didn't just churn out a sequel as most software houses do. And the answer was that they wanted to raise the bar a lot with each new installment, that they were not interested into "more of the same".
It could take some time before we see a HL3, but it's one of those rare games that is already sold for me :)
