You've combined steps... draw a shape, save as font is actually two separate processes requiring two separate apps.
Check through the unicode characters first. The majority of the time, you'll probably just need to use a basic shape (or a "primitive") like circles or squares. Unicode has both solids and outlines of these shapes (and a ton of other stuff). Everything 3D in this video (https://www.youtube.com/watch?v=r-BEqBaYJ9k) are unicode characters (except the "chairs"). The beginning demonstrates combining characters to build a new shape.
If you need some shape you cannot find in the unicode character set, then the next step to take is to develop the shape and save it as an SVG file.
To be able to export shapes from Motion as SVG paths is high on my wishlist!!! Anybody else??? Provide Motion Feedback!
There are numerous applications which you can use, some are freely available, others are not. iVinci Express is free from the App Store, but there are several to choose from that are not too costly. These apps are like Adobe Illustrator to some degree, they are apps you can use to create vector graphics. Anyway, it really doesn't matter much as long as the saved SVG file is 100% compatible with the last step apps you'll need. One, as Ben said is mObject. mObject is an excellent (and if you're really interested in 3D in Motion, very recommended) plugin for Motion/FCPX from MotionVFX.com. The other is to convert the SVG path to a Font. http://icomoon.io/app is free to use and you use it directly from your browser. You upload your saved SVG file to the site. You can upload several at a time. You can also include the icons they have available on that site into a font—just select which ones you want to include. You might find the shapes you're looking for within the collection of icons available. These are all open source and available for commercial use. Once you've uploaded your SVG files, they will appear in an "Untitled Set". You can select which ones to include in your font and go through the hundreds (thousands?) of other icons to include in the font you want to build. When you click the Generate Font button at the bottom, you should also click on the Gear icon/Preferences and give your custom font a Custom Name (you can create as many fonts with different names as you like.) Then click the download button at the bottom of the page. Find your newly created font and install it with Font Book. I recommend creating a new custom "Collection" in Font Book and dragging the font into the collection you created. It will make it much easier to find in Motion as Motion supports Collections for fonts. To use your custom shapes in Motion, all you really need to do is start a Text Object in the canvas, open Font Book, find your shape, select it, copy it and back in Motion, Paste. Motion (usually) automatically changes the font to the selection in Font Book for you so your newly added shape should be ready to go. Go into the appearance panel and turn on 3D (you can start with 3D text if you click/hold the T button and select 3D Text from the dropdown menu under the canvas.)
You normally will resize your text(shape) larger with Text > Format > Size and center it with the Baseline. I recommend applying guides at 0,0 and centering your text in the canvas (both Properties and Center Alignment) before adjusting Baseline. You'll adapt quickly when you see how it works. Select the 3D Transform tool to get the bounding box with the little blue control dots around the edges -- they'll guide you to aligning quickly with the guides. This little bit of workflow will help out a great deal while you "inspect" your work (add a camera and use the onscreen camera controls to maneuver your object around. Afterwards, you can readjust the anchor point and place the object anywhere in space you like.
As you inspect the 3D text tools in Motion, you'll discover that you can use other "texture" sources (usually in "custom" options.) You can use the same images as "bump maps" to make a plain image a realistic texture. There are an enormous amount of options available in this regard because you can use emitters, replicators, and animations as "textures". Anything you can creatively design, you can apply to 3D text in Motion. My basic advice for this is:
1) create a "texture" group for your design build. 2) "fix" its size (make the group 2D Fixed Resolution and size it to fit the graphics; example: a 2000 x 2000 square [doesn't have to always be any specific size -- only if you just want to keep "gaps" trimmed out] -- otherwise the gaps will be transparent, which can be cool too); 3) [IF using a drop zone] select the texture group and type K to create a Clone Layer. Name the Clone Layer the texture name — otherwise use the group as the "texture". 4) Apply the Clone Layer/Group texture to the source wells (remember to Add Layer > Distress > Custom Bumps if necessary.); 5) Make sure your scales, rotations, positions you supply [in the Placement section] to one part is consistent across all the textures you apply as you Add Layers.
If your design includes a VIDEO drop zone, never create the project as a Title (even though it's technically text), Effect or Transition—they won't work. ONLY create Generators for FCPX (that use a drop zone for video). If your drop zones will only contain stills, then it doesn't matter what kind of effect it is. A Video drop zone must be in a "texture group" and Cloned in order to work (do not ask me why... it's one of *those* mysteries...)
You can check these videos out: https://www.youtube.com/playlist?list=PLjOCgGxNnAk4W3gTBk71qNsi8Yj2KULoh
You'll find some with Project files you can download and Fonts you can use and they'll give you an idea of the kinds of things you can do with 3D in Motion (and even though it's ALL text, I don't use it as 3D text all the time -- I'm going for real 3D Models.) [Beware of filters that cause rasterization - they'll screw up "layer order" appearance of 3D objects as they rotate around in space,]
Have fun!
