In this paper, we propose a thermo-optic reconfigurable three-mode (de)multiplexer based on an asymmetrical horizontal three-waveguide directional coupler that includes two identical single-mode waveguides and a three-mode waveguide. Over the whole wavelength range of 1540-1560 nm, and for the TE (TM) polarization, our typical fabricated device with polymer material shows coupling efficiencies as high as 94% (93%) and 93% (92%) for the mode conversions of L P 01-L P 11a and L P 01-L P 11b , with the heating powers of 53.57 mW and 71.19 mW, respectively. Our proposed device can be employed in the fields of reconfigurable mode-division multiplexing systems.