That's exactly right. And, everybody has the same problem.
3D and 2D don't really mix. You either have to limit your view and arrange layers in a specific order, or use all 3D elements (you will lose the ability to get reflections of other 3D elements in a scene — the only reflections you get are with the Environments.)
It's not difficult to create a 3D plane. There are plenty of rectangular shapes in the Unicode character set. All you need to do is, at the text prompt in the canvas, type Control - Command - Space and it will call up the unicode character viewer (it takes abut 2-3 seconds... be patient.) You should be able to find characters like: ▂, ▃, ▄, ▅, ▆, ▇, ▎, ▍, ▋, ▊, ◼as well as a vast array of other characters. Just double click the character and it will be inserted at the carat. You can size characters up to about 5000 (Motion will tell you if you've gone too far.) If you're going to be using the unicode characters frequently, I recommend Add to Favorites (top right hand side under the current selected character display) so you can easily find it again from the Favorites category (left hand column list.)
