In this paper, I develop a non-linear theory of tightly wound (highly twisted) eccentric waves in astrophysical discs, based on the averaged Lagrangian method of Whitham. Viscous dissipation is included in the theory by use of a pseudo-Lagrangian. This work is an extension of the theory developed by Lee & Goodman to 3D discs, with the addition of viscosity. I confirm that linear tightly wound eccentric waves are overstable and are excited by the presence of a shear viscosity and show that this persists for weakly non-linear waves. I find the waves are damped by shear viscosity when the wave become sufficiently non-linear, a result previously found in particulate discs. Additionally, I compare the results of this model to recent simulations of eccentric waves propagating in the inner regions of black hole discs and show that an ingoing eccentric wave can be strongly damped near the marginally stable orbit, resulting in a nearly circular disc, with a strong azimuthal variation in the disc density.