---

We all know valve doesn't like the number 3 for some reason, but what we do know, is that they like making extra parts to number 2. So hey, listen to the reasons, you might get convinced!

---

HOW WHEATLEY COULD MAKE A THIRD:
He's in space, and sure he's gonna keep floating in one direction, but what if he hits a rock? He could float back to earth and land on something safe. Then someone could give him back to aperture and he would be BACK IN BUSINESS! Ready to attempt to kill you, again!

HOW CHELL'S PARENTS COULD MAKE A THIRD:
They're somewhere, and if they gave her directly to aperture, chances are, they're somewhere in aperture.
GLaDOS could get bored and, Chell's parents are in action!

HOW GLADOS COULD MAKE A THIRD:
Sure, he gave up on Chell, but there's still thousands of other people for testing. And 2 robots. GLaDOS could make them test more and more.

HOW CHELL COULD MAKE A THIRD:
GLaDOS didn't take the portal gun of her, so she still has one, so she can do what ever she wants! She could try to get rid of aperture laboratories (because she nearly got killed three times: Suffocating, Exploded. Twice.) so nobody would have to suffer.