Thermodynamic optimizations of the Mo-Nb, Mo-Ta, and Nb-Ta binary systems were performed by considering the experimental phase diagram and thermodynamic data available from the literature. The Mo-Nb-Ta ternary system was then synthesized from the descriptions of the binary sides. The calculated thermodynamic properties as well as phase diagrams for the binary systems agree well with the experimental ones. In the case of the ternary system, several isothermal sections were calculated by extrapolation from the three boundary systems. The model-predicted liquidus temperatures in the ternary system are in reasonable agreement with the experimental data. The computed liquidus projection and a set of model parameters describing the Gibbs energies of the assessed phases are also presented.