An effective triplet potential is proposed. This triplet potential expresses the effects of Axilrod-Teller three-body interactions on a pair of molecules, and is given as a simple function of density and the distance between the pair of molecules. The computer time for Monte Carlo simulations of fluids with both a pair potential and the effective triplet potential is almost equivalent to that of fluids with only the pair potential. By using an accurate pair potential together with this effective triplet potential, Monte Carlo simulations are performed to estimate the thermodynamic properties of argon, and the results are compared with the experimental properties, the saturated vapor and liquid densities, the heats of varporization and the second virial coefficients. The comparisons show that the simulation results for fluids with only the pair potential give poor agreement with the experimental data, especially for the heats of vaporization where the simulation results give very large deviations. However, the simulation results for fluids with both pair potential and triplet potential give very good agreements with the experimental data of argon.