user2264: What I'd like to do
I'm trying to draw a 2D smiley face with the pen tool on a 3D layer...and then change it to a frown (or a sad face). I was able to do this in as 2D layer...but I'd need help on doing it as 3D layer.
Why I'm using 3D Layers in the first place
I'm using a camera (attached to a null layer) to zoom in and out of certain images I have in my animation (which are imported from illustrator). I've realized that you have to have 3D layers if you're gonna pan and zoom with the camera tool!
What's the Problem
It's close to impossible to draw using the pen tool if the pen layer is 3D. Even if I do get something drawn, I can't figure out how to animate it as a 3d layer. 
Any tips?

user2197: 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

user1950: Animating a shape layer with the pen tool is the same for 3D layers as it is for 2D. One problem is that if you're moving paths around on a layer that is not parallel with the view then you have pespective issues, or parts of your layer might go out of view.
To get around this you can create a new camera that is parented to the shape layer - best to do this before transforming the shape layer. Then you can switch to that camera to get a straight-on view of the shape layer.

Here we have the shape layer with animated paths and a camera called "shape cam" that is parented to it, as well as the scene camera called "active cam". 
If we switch to shape cam using the viewport control at the bottom of the comp window we see a flat-on view of the shape layer, for easy editing. A handy tip is that once you've switched from one view to another, to switch back to your original view you just hit escape, so you can toggle back and forth with ease. 

You can also set up another view (view > new viewer) and lock one on the shapeCam (the padlock in the view tab) and one on your scene camera, so you can edit your shape layer in one view in mock-2d and see it from your 3D camera at the same time. Here we see the shape layer at the next keyframe, where the paths have been edited, from both the shape cam view and the scene camera view.

An advantage of using shape layers is that they are not raster layers like an adjustment layer with the stroke effect. You can zoom right in on them without the pixels ever getting nasty


2264: <h2>What I'd like to do</h2>

<p>I'm trying to draw a 2D smiley face with the pen tool on a 3D layer...and then change it to a frown (or a sad face). I was able to do this in as 2D layer...but I'd need help on doing it as 3D layer.</p>

<h2>Why I'm using 3D Layers in the first place</h2>

<p>I'm using a camera (attached to a null layer) to zoom in and out of certain images I have in my animation (which are imported from illustrator). I've realized that you have to have 3D layers if you're gonna pan and zoom with the camera tool!</p>

<h2>What's the Problem</h2>

<p>It's close to impossible to draw using the pen tool if the pen layer is 3D. Even if I do get something drawn, I can't figure out how to animate it as a 3d layer. </p>

<p>Any tips?</p>
 2197: <p>The way I normally go about drawing paths in AE is by using the stroke effect with masks</p>

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

<p><img src="https://i.stack.imgur.com/MF5pX.png" alt="After Effects Smiley Screenshot">
<a href="https://i.stack.imgur.com/MF5pX.png" rel="nofollow noreferrer">http://i.stack.imgur.com/MF5pX.png</a></p>

<p><strong>Note:</strong> </p>

<p>For some reason the stroke effect doesnt work on <em>open paths</em> when there is <strong>more than one mask</strong> 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. </p>

<p>Have a look at what I did: <a href="http://www.filehosting.org/file/details/334754/Smiley_Face.aep" rel="nofollow noreferrer">http://www.filehosting.org/file/details/334754/Smiley_Face.aep</a></p>
 1950: <p>Animating a shape layer with the pen tool is the same for 3D layers as it is for 2D. One problem is that if you're moving paths around on a layer that is not parallel with the view then you have pespective issues, or parts of your layer might go out of view.</p>

<p>To get around this you can create a new camera that is parented to the shape layer - best to do this before transforming the shape layer. Then you can switch to that camera to get a straight-on view of the shape layer.
<img src="https://i.stack.imgur.com/l2fkX.png" alt="two camera setup">
Here we have the shape layer with animated paths and a camera called "shape cam" that is parented to it, as well as the scene camera called "active cam". </p>

<p>If we switch to shape cam using the viewport control at the bottom of the comp window we see a flat-on view of the shape layer, for easy editing. A handy tip is that once you've switched from one view to another, to switch back to your original view you just hit <kbd>escape</kbd>, so you can toggle back and forth with ease. 
<img src="https://i.stack.imgur.com/wtkOI.png" alt="enter image description here"></p>

<p>You can also set up another view (<strong>view > new viewer</strong>) and lock one on the shapeCam (the padlock in the view tab) and one on your scene camera, so you can edit your shape layer in one view in mock-2d and see it from your 3D camera at the same time. Here we see the shape layer at the next keyframe, where the paths have been edited, from both the shape cam view and the scene camera view.
<img src="https://i.stack.imgur.com/YrYBu.png" alt="enter image description here"></p>

<p>An advantage of using shape layers is that they are not raster layers like an adjustment layer with the stroke effect. You can zoom right in on them without the pixels ever getting nasty
<img src="https://i.stack.imgur.com/aTj7l.png" alt="enter image description here"></p>
