Compression factors for (methane + ethane + carbon dioxide) where x(CH4) approximate to x(C2H6) approximate to x(CO2) were measured using a direct method in the temperature range 283.15 K to 333.15 K and at pressures up to 7.5 MPa. Mixture second virial coefficients were calculated, using correlations available in the literature for both pure component and second unlike-interaction virial coefficients, and compared with the experimental measurements. Mixture third virial coefficients were calculated, using pure component virial coefficients from the GERG correlations and cross third virial coefficients extracted from earlier experimental data on binary systems using the direct method, and compared with the experimental values. The third cross virial coefficient, C-123 was calculated from previous binary data and also from the ternary data of this study.