Erm, select the empty box next to the file name of the layer off to the left of the timeline, a little 3D cube should appear. This means the layer is now 3D. (If you don’t see any boxes to select, hit F4). By using the blue arrow (or one of those) in the preview screen, you can now give your object depth in Z-space…likewise you could just change the z-position under the position values.
Is this what you seek to accomplish?
Best,
Hunter