I was pondering that eariler today, and figured that if you made the center wheel flat, it would help keep it centered when upright. Also, having a "bar" stick out from the sides would help when turning to keep it from rolling over itself...... It would take a lot of precision to keep the CM in the middle.... but I think its possible. Maybe have a gyroscope help keep the weight under control? I'm not experienced in using them, but maybe if it senses it out of balence, it can adjust the weight (using a large weight on a slider) to compensate for the angle? Would require a lot of calculations though.....
This is an image I threw together in CAD showing the "flat bottom" center wheel and the bars on the side to help it when turning..... The red dots symbolize the CM.....
