The scattering of plane surface waves by bottom undulations in a three-layer channel of different constant densities is investigated here by using a simplified perturbation analysis. In such a three layer fluid there exists waves of two different modes propagate along each of the interfaces. In the process of obtaining solution for the problem a Fourier transform technique is employed to derive the first-order corrections of the reflection and transmission coefficients in terms of integrals involving the shape function representing the bottom elevation. For sinusoidal undulations, these coefficients are plotted graphically to illustrate the energy transfer between the waves of different modes induced by the bottom undulations. It is shown that the scattering phenomena effected by the change in different parameters, viz, the porous effect parameter, the number of ripple present at the bottom and the depth of the lower layer.