This paper presents a new three-dimensional (3-D) nonlinear optimal guidance law to control the impact angle in maneuvering target interception. This approach formulates the optimization problem in a relative frame that enables the derivation of the optimal solution without using any linearization assumptions. The key advantage of the proposed 3-D guidance law lies in its exact nonlinear nature and hence the tolerable initial heading error is much larger than that of the linear optimal guidance laws. The connection with existing optimal guidance laws is analyzed and the results reveal that the proposed approach unifies previous linear solutions. The working principle and characteristics of the guidance command are also investigated to support the implementation. Extensive numerical simulations are performed to demonstrate the working of the proposed approach and its advantages.