3D differential geometry

I'm going to present something about 3D GR in some days.  However, I keep reading that 3D is conformally flat, but I don't see why.  Wikipedia says that the Weyl tensor vanishes, but that's apparently not enough for 3D.  What is the significance of (what wikipedia calls) the cotton tensor, and why does it also have to be 0?

I should probably go read some more papers, but if any of you guys have some insight, that'd be nice.

>>5
OOOOHHHH,  I get it now!

The answer is 4.