The growing utilisation of non-linear loads poses significant challenges to power quality in electrical grids. Non-linear loads, such as three-phase diode rectifiers, produce substantial harmonics, necessitating effective control methods to mitigate harmonic distortions. The implementation of active power filters (APF) offers significant potential for enhancing harmonic compensation performance. Accordingly, this research introduces a non-singular fast terminal sliding mode controller (NFTSMC) integrated with proportional multi- resonant (PMR) based control for a three-phase APF. The primary objective of this approach is to achieve rapid and precise tracking of reference currents while simultaneously injecting the desired currents into the electrical grid. Furthermore, to reduce harmonics generated by high-switching-frequency inverters, incorporating an LCL filter with an active damping strategy is essential. The PMR controller generates capacitor voltage references, ensuring zero steady-state error in the grid current and facilitating the implementation of the active damping method. Simulation results demonstrate the ordinary sliding mode controller (SMC) by achieving significantly lower total harmonic distortion (THD). Additionally, it exhibits superior dynamic response and robustness presence of uncertainties.