Pressure and dielectric virial coefficients and compression factors of three mixtures of the CH4-C2H6-N-2 system, three mixtures of the CH4-CO2-N-2 system and three mixtures of the CH4-C2H6-CO2 system have been investigated experimentally using the dielectric constant method. Precise (+/-5 ppm) measurements of the dielectric constant epsilon as a function of the pressure P along one isotherm T are combined with the first three dielectric virial coefficients A(epsilon), B-epsilon and C-epsilon of each mixture to obtain accurate values of the molar density p. The compression factor Z = P/(rho RT) was obtained from the measured values of epsilon, P and T. We report the values of Z at 298.15 K up to 12 MPa for the nine mixtures. The first three dielectric virial coefficients, and the second and third pressure virial coefficients for most of the mixtures agree with the calculated or literature values within the limits of uncertainty. Experimental values of Z are compared with the calculated values from the GERG equation. The mean deviation in Z is of the order of 0.11% for all the mixtures. This is quite similar to the expectation accuracy of 0.1% for the GERG equation.