We present a new scalar potential method for computing the electromagnetic variables in multilayer slab conductors, excited by waves of arbitrary shape. In this method a large number of layers can be modeled separately, and it is possible to consider the arbitrary shape exciters in between the conductor layers. We use the scalar potential method to solve the Helmholtz equation in three dimensions. For the solution of this problem, two-dimensional fast Fourier transform and exponential functions are used with scalar potential variables. We verify the method by comparing the results of an example problem solved both by the new method and the finite-element method.