By considering the effect of high-field-side cutoff, the conventional Budden model has been extended to Triplet model. In this model, the reflection coefficient, transmission coefficient and mode conversion (MC) coefficient of the fast wave for a single evanescence region can be derived through using phase-integral method. Furthermore, numerical calculation of MC coefficient for double-ion species and three-ion species have been done. In the case of double ions, the result is consistent with Kazakov's work. In addition, as an example of three ions plasma, (H, 3He)D plasma in tokamak EAST, simulations of the dependence of the MC efficiency on the magnetic field, microwave frequency and minority concentration for different antenna phasings are carried out. The results show how to choose proper phasing to reach an optimum MC efficiency. This result may provide a reference to improve ICRF heating efficiency.