For a first robot, I would recommend you definitely use differential drive and not go for 4 wheels. A sumo bot will need its powered wheels to be centered under as much of the robot's mass as possible for maximum traction; you might actually only want two wheels! You also would't want to add a third motor in any case, as the benefit would be minimal compared with the added complexity in your code for moving the robot. I have worked on a sumo robot project which ended up using two motors, one wheel each, and the front was just a super-sharp wedge that got underneath enemy robots and scooped them up so they lost traction. (That was radio controlled though.) If you need anything else, let me know.