If you haven't already created a line in your After Effects project, here's how. Go to "Layer" > "New" > "Solid." A toolbar will pop up to allow you to choose some details of your solid layer, including color. Once you've decided on these details, click "OK." In your toolbar, click the rectangular mask tool. Click and drag within your composition on top of your new Color Solid layer, and let go once you've made a shape that pleases you. Using the arrow tool, you can click on the corners of the mask to adjust them. In your time line, press the "M" key on your keyboard until "Mask 1" appears under your Color Solid layer. Here, you can make adjustments to the mask, such as feathering the mask so that the border of the line is gradual instead of immediate. If you'd like the line itself to be three-dimensional, a good way to do this is to apply the "Bevel Layer" effect to make the line protrude from the canvas.
Just above your time line, there are a few options that will affect all layers in your time line. One of these options is a button that looks like a three-dimensional box. Click on this to make any layer within your composition a three-dimensional layer. Now, go to your time line and your Color Solid layer. There will be at least three columns of unchecked boxes. One of those columns is under a three-dimensional box logo that looks just like the one you clicked a moment ago. Click under that column, within the unchecked box that sits to the right of your Color Solid layer. Your Color Solid layer, which you've turned into a line, is now a three-dimensional layer that can be moved not just up, down, right and left, but also forward and backward.
Click on your Color Solid layer. On your keyboard, press "P." The word "Position" will appear under your Color Solid layer because you previously selected it. Three numbers will appear. The first is the position of your layer in X-space; the second, its position in Y-space; the third, its position in Z-space. You can adjust the position of your line either by clicking and dragging left or right on these three numbers, or by clicking on the layer itself in your canvas and manually moving it.
Determine the starting point of your line. In your time line, move your cursor all the way to the left. Underneath your Color Solid layer, to the left of the word "Position," click on the stopwatch logo. This creates the first keyframe in your time line. Now, determine how long you would like your line to move. Move your time line to the right until you've reached the exact frame in the time line at which you'd like your line to stop moving. Now, determine the position at which you'd like your line to end. Move your line to this position. Another keyframe at this exact frame will be created for you. Because you have two keyframes in two different spots in the time line, your 3D line will gradually move from the original position at the first frame to its end position on whatever frame you have chosen.
Position is the primary way to move your line in 3D space. However, you can apply more than one type of keyframe to your line at once. Experiment with type of motion, such as rotation, to make your line rotate while it moves forward and back. You can also apply a motion blur for added realism by clicking on the unchecked box underneath the circular motion blur icon. Once you are happy with the motion of your line, click "Composition" > "Add to Render Queue." Go to your Render Queue and render your new graphic.