We consider the coupling between three photonic crystal waveguides as a multimode interference system and show that the dispersion curves of the eigenmodes intersect or almost intersect. Degenerate modes appear in the system. Due to the strong coupling between the degenerate modes, the modal patterns of the guided modes are changed obviously. We find that the power distribution of output light waves of different frequencies is different by selecting the coherence length. At the crossing point, the multimode interference is deprived and power is confined to its input direction without observable transferring to other photonic crystal waveguides. On the basis of these, a wavelength de-multiplexer or multiplexer is designed.