The motion of a wheelchair is different from any other vehicle. It needs control in three dimensions: the longitudinal, the lateral, and the pitch direction. This paper takes this point into consideration and provides a 3-D control of a wheelchair. We focus on the control of a push-rim power-assist wheelchair. The main objective of this paper is to provide a wheelchair rider with 3-D assistance that guarantees power and safety. To this end, three kinds of assist controls are proposed: the disturbance attenuation controls are designed for the longitudinal and lateral directions while the tip-over prevention control is designed for the pitch direction. These controls for three directions can be integrated by appropriately taking advantage of system configuration. We demonstrate that all these controls can work independently for each purpose and we show experimental results.