A modified effective index method (MEIM), which correctly describes, in a 2D case, both the phase and the group indexes in a 3D photonic wire waveguide, is introduced in the Letter. The MEIM utilizes the combined index profile with two spatial parameters. The central part with wire index has the width of nearly a wire height, and it is responsible for the group index. The basic part has the width of 3D wire and the smaller refractive index N(b). Numerical experiments by finite-difference time-domain method of Fabry-Perot and asymmetric Mach-Zehnder interferometers constructed by silicon wires prove that the MEIM gives an excellent agreement (about 2%) with test 3D simulation in comparison with an about 40% error for the effective index method.