Uses this site as a reference, http://www.denverplants.com/foliage/html/AraFal.htm
, and just the plant grow. You have 3 leaves. Let it get almost as big as the picture (on the reference link) or when the new growth slows down. When it gets all healthy and has alot of leaves, cut it back, cut off the new buds and some of the new growth. Dont take more than 25-50% or so of the leaves and branches. Let it grow back and repeat. As you keep cutting it back like this, the trunk and branches will keep getting thicker. In short, what you are doing is rather than letting the plant grow and focus energy on the new growth, you cut that off and it focus its energy on existing growth, so it grows thicker. Depending on how fast the plant grows, it might take you 5-50 years to get your desired results. Patience is the key and dont over water. It shouldnt matter what kind of pot you have, as long as it can grow in what you have.

Here is a link that might be useful: reference

Dizygotheca (your plant) does not make fat trunks - but if you can follow bonsai techniques otherwise, you can still get an interesting looking tree out of it.

cool, thanks for the quick response! I also have an Aralia Balfour that is about a foot in a half tall with two main trunks. Could I thicken up its trunks the same way?
Whitney