Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are recognized as the most promising magnetar candidates, as indicated by their energetic bursts and rapid spin-downs. It is expected that the strong magnetic field leaves distinctive imprints on the emergent radiation both by affecting the radiative processes in atmospheres of magnetars and by scattering in the upper magnetospheres. We construct a self-consistent physical model that incorporates emission from the magnetar surface and its reprocessing in the three-dimensional (3D) twisted magnetosphere using a Monte Carlo technique.The synthetic spectra are characterized by four parameters: surface temperature kT, surface magnetic field strength $B$, magnetospheric twist angle $\Delta\phi$, and the normalized electron velocity $\beta$. We also create a tabular model (STEMS3D) and apply it to spectra of magnetars.