This paper addresses moderately large vibrations of immovably supported three-layer composite beams. The layers of these structural members are elastically bonded, and as such, subjected to interlayer slip when excited. To capture the moderately large response, in the structural model a nonlinear axial strain-displacement relation is implemented. The Euler-Bernoulli kinematic assumptions are applied layerwise, and a linear interlaminar slip law is utilized. Accuracy and efficiency of the resulting nonlinear beam theory is validated by selective comparative plane stress finite element calculations. The outcomes of application examples demonstrate the grave effect of interlayer slip on the geometrically nonlinear dynamic response characteristic of layered beams.