3-Wheeled Robots may be of two types:
1-Differentially steered (2 powered wheels with an additional free rotating wheel to keep the body in balance)
2-Two wheels powered by a single source and a powered steering for the third wheel.
In the case of differentially steered wheels,the robot direction may be changed by varying the relative rate of rotation of the two separately driven wheels. If both the wheels are driven in the same direction and speed, the robot will go straight. Otherwise, depending on the speed of rotation and its direction, the center of rotation may fall anywhere in the line joining the two wheels.
The center of gravity in this type of robot has to lay inside the triangle formed by the wheels. If too heavy of a mass is mounted to the side of the free rotating wheel, the robot will tip over.
See All Types Of Robots - By Locomotion