Based on the double Fourier integral representation for steady viscous ship waves, which involves a generic amplitude function and a complex dispersion relation, the phase function for three-dimensional ship waves in the far field was asymptotically derived by means of the Lighthill two-stage scheme combining the Cauchy residue theorem and the method of steepest descents, and the effect of viscosity on wavelengths was numerically investigated with the Newton iteration method. It is found that the transverse and diverging waves are elongated due to the presence of viscosity and the latter is more heavily affected and that the phase differences between the viscous transverse and diverging waves are larger than those of the corresponding inviscid waves.