Maybe.
I think the people at Valve would like to finish it, but they might have other things they want to do too.
Also, they are very very aware that they've made a popular game series and then taken a long time to finish it, so anticipation has climbed very high.
And I would be afraid when approaching such anticipation that I could ever match it.
I wouldn't want a Mass Effect 3 to happen again if I was making Half Life 3.
inb4It'sHL2Ep3, so no