The nonlinear mechanics of three-layered microcantilevers under base excitation is investigated numerically. Employing the modified version of the couple stress-based theory, together with the Bernoulli-Euler beam theory, the potential energy of the three-layered microsystem is derived, while accounting for size effects. Obtaining the kinetic energy, the equations of motion in the longitudinal and transverse directions are derived via Hamilton's principle. Application of the inextensibility condition reduces the two equations of motion to one nonlinear integro-partial differential equation for the transverse oscillation, consisting of geometrical and inertial nonlinearities. The nonlinear equation of partial-differential type is reduced to set of equations of ordinary-differential type through use of a weighted-residual method. Solving the resultant set of discretised equations via a continuation technique gives the frequency-amplitude and force-amplitude responses of the microsystem. The nonlinear response is investigated for different layer composition and different layer thicknesses. The effect of small-scale parameter, as well as base excitation amplitude, is also examined.