I don't know if you've started shaving or not, but I notice that within a week after I don't shave, such as when I'm on vacation, it starts to grow past an inch. It becomes too long to be considered close - shaven, and too short to be an acceptable beard. If that happens in a week, then imagine what would happen in 3 years of not shaving.
Let's also consider:
- He would have no access to materials to make fake hair.
- He has no reason to make hair.
- He has no hair with which to make a fake beard and grown head of hair.
According to the Google, the average rate of hair growth is about 6 inches per year. Which means if Zaheer's hair didn't grow particularly fast or slow, we'd expect him to have about a foot & a half of hair after 3 years.
Yeah...I'm gonna second what Neo said. He didn't look that unduly scruffy for having been in an oubliette for the past three years; and he looked less scruffy than he did during his jailbreak. Nothing out of the ordinary.