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The hybrid three-arm coupler, which consists of the middle long range surface plasmon polariton (LRSPP) waveguide and two outside conventional dielectric waveguides, is proposed and analyzed numerically with finite element method. The characteristics, including coupling length, coupling loss, extinction ratio and frequency dependence, are discussed in detail with different structure parameters. Compared with the two-arm hybrid coupler, the coupling loss of the three-arm hybrid coupler based devices can be reduced, especially with wider middle LRSPP waveguide. Extra transmission loss on the metal strip can be avoided because of the lossless input and output arms. Furthermore, different from conventional three-arm dielectric coupler, we find that the three-arm hybrid coupler with asymmetric structure can reach a much higher extinction ratio by introducing a middle arm offset. Since the middle arm can only transfer the TM mode from one dielectric arm to the other one within the TM mode coupling length, this structure is promising for realizing integrated polarization splitter or other functional devices.