New experimental data on the pressure-density-temperature relationship of liquid octafluoropropane and two near azeotropic mixtures R218/R134a/isobutane and R125/R134a/n-butane are reported, obtained with a constant volume apparatus, which makes possible density measurements at subzero temperatures. The data cover temperatures from (203 to 353) K and at pressures up to 40 MPa for octafluoropropane and up to 50 MPa for the mixtures. The combined uncertainty at the 0.95 confidence level of the resultant density data is estimated to be 1 kg.m(-3), that is, about 0.0008 rho for the octafluoropropane density rho. The root-mean-square deviation of the obtained data for octafluoropropane from a Helmholtz free energy equation of state fitted to them is 0.56 kg.m(-3) (0.0004 rho). For both mixtures, about 93 % of the experimental density data points lie within the interval +/- 0.002 rho from the Lemmon-Jacobsen Helmholtz energy model for these mixtures.