Here is a 3WD mini mobile robot kit that lets you experiment with Arduino. The Nexus Omni Wheel Mobile Robot is capable of moving in any direction with a max speed of 0.6m/s. It has a load capacity of 5kg.
The robot has 3 omni wheels, micro-controller, IO expansion board, DC motor with encoder, and ultrasonic sensors. You can program the robot with C and C++.