Wow! Even at 3' tall? Alright. Well, maybe I shoild have gotten the 5-6 footer.
Well, it is not much a matter of height, but of thickness and the taper of it. The most important things you should probably look for when buying a tree or pre-bonsai material is in first place a big, fat and powerful trunk. One that progressively becomes thinner while going upwards. Or a trunk with a lot of movement and distinctive features. Then a good nebari (roots) is highly desirable. Roots, also strong and powerful that sink firmly within the substrate and flare out from the central axis of the tree.
This is not casual. This is chosen this way, because these two features are the hardest to develope. Consequently, these also are the ones that take longer to properly achieve.
Alain