We approximate the nonlinearly coupled transverse-axial motions of an isotropic elastic plate with three nonlinearly coupled fundamental oscillators, and show that transverse motions can be decoupled from in-plane motions. We demonstrate this decoupling by showing analytically and numerically the existence of a global two-dimensional nonlinear invariant manifold. The invariant manifold carries a continuum of slow, periodic motions. In particular, for any motion on the slow invariant manifold, the transverse oscillator executes a periodic motion and it slaves the in-plane oscillators into periodic motions of half its period. The spectrum of the in-plane slaved motions consists of two distinct harmonics with frequencies twice and quadruple than that of the dominant harmonic of the transverse motion. Furthermore, as the energy level of motion on the slow manifold increases the frequency of the largest harmonic of the in-plane motions approaches the in-plane natural frequencies. This causes the in-plane oscillators to oscillate in pure compression.