We present a three-dimensional motion planning framework for a coupled-mobility snake robot that incorporates centipede-like crawling and a variety of rolling gaits. The snake robot is equipped with a number of feet on its underside that enable it to crawl over and around obstacles. Due to its flexible body structure, the snake also retains the ability to move without the aid of its feet, through internally induced bending motions - in this paper we focus specifically on a class of lateral rolling gaits. The motion planning framework is based on fitting the snake robot's kinematic structure to a three-dimensional spline curve passing through prescribed interpolation points. In the case of linear crawling, the curve defines a path to which the snake is fitted as it crawls forward. For a rolling gait, the curve is used to define the shape of the snake as it repeatedly rolls about its own center axis. The framework outlined in this paper can be adapted to a wide range of modular snake robots. Numerical results demonstrating the computation of joint angle trajectories for two different rolling gaits are presented.