Looks like a great start!
Few comments:
1. Why are the gearboxes not symmetrical? It doesn't seem like your cbore'n the holes so you could assemble two different gearboxes with the same components. Similar to the gearbox below.
Poofs PTO guy from 2013 -
http://puu.sh/907Xp/226027b712.jpg
2. It seems as your direct driving the center wheel, why not use VersaPulleys (Larger Diameter) to save cost and load the belts less?
3. Do you have a Versahub + bearing on each wheel? I can't see if there a bearing supporting the other end of the wheel.
4. Also outer wheels could be dead vs live. You could get rid of the versahub and move bearings into the wheel/pulley.
5. Any reason for not jumping down to 4" wheels?