In the moderate magnetic field of TCV (1.5 T), the recently installed X3 system (3 gyrotrons, 118 GHz, 0.45MW each, 2 s) broadens the operational space with the possibility of heating plasmas at high density, well above the cutoff density of the X2 system. To compensate for the significantly weaker absorption coefficient compared to the absorption of X2, the top-launch injection allows the ray path to maximize along the resonance layer thereby maximizing the optical depth. To maintain the maximum absorption in plasma discharges with a dynamic variation in both density (refraction) and temperature (relativistic shift) a real-time control system on the mirror injection angle, based on a mirror modulation technique (synchronous demodulation), has been developed and successfully tested on TCV. Comparisons of the absorption calculated with the TORAY-GA ray-tracing code and the ECWGB beam-tracing code, which includes diffraction effects, are presented. An experimental study of the X3 absorption versus plasma density in an L-mode plasma shows that with a total injected power of 1.35MW full single-pass absorption is reached with a significant fraction of the absorbed power associated with the presence of suprathermal electrons. Compared with ohmic/low-power-heating of ELMy H-modes, it has been possible to enter into a different ELMy regime with an injected power of 1.35MW.