animating a line that is a 3d layer in after-effects
The way I normally go about drawing paths in AE is by using the stroke effect with masks


Create an Adjustment Layer
Create Mask Paths for the smiley face
Add the Stroke effect to the Adjustment Layer 
Make the Adjustment layer 3D and then position it where you want
Animate the Mask Path to animate the mouth



http://i.stack.imgur.com/MF5pX.png

Note: 

For some reason the stroke effect doesnt work on open paths when there is more than one mask on a layer. So I put the eyes on one Adjustment layer, and made a second Adjustment layer for the mouth. Then I turned OFF 3D on the layers, PreComped them together, and made the PreComp a 3D layer. 

Have a look at what I did: http://www.filehosting.org/file/details/334754/Smiley_Face.aep