A compact vertical coupler is proposed and optimized based on the multi-phase matching for three-dimensional (3D) mode (de)multiplexing [(De)MUX]. The multi-phase matching conditions can be achieved for the operating TE₂, TE₁, and TE₀ modes of the subwavelength grating-based waveguide. The coupling strength can be significantly increased due to the subwavelength structure. The proposed 3D mode (De)MUX is optimized by utilizing the 3D full-vectorial finite difference time domain method. The optimized mode (De)MUX can achieve the coupling lengths of only 0.25 and 1.75  μm for demultiplexing TE₁ and TE₂ modes. The insertion losses of the TE₂, TE₁, and TE₀ modes are only 0.25, 0.27, and 0.13 dB, respectively, and 3-dB bandwidths are calculated to be 364, 203, and >300  nm, respectively.