We consider three-flavor chiral perturbation theory (χPT) at zero temperature and nonzero isospin (µ I ) and strange (µ S ) chemical potentials. The effective potential is calculated to next-to-leading order (NLO) in the π ± -condensed phase, the K ± -condensed phase, and the K 0 /K 0 -condensed phase. It is shown that the transitions from the vacuum phase to these phases are second order and take place when, |µ, respectively at tree level and remains unchanged at NLO. The transition between the two condensed phases is first order. The effective potential in the pion-condensed phase is independent of µ S and in the kaon-condensed phases, it only depends on the combinations ± 1 2 µ I + µ S and not separately on µ I and µ S . We calculate the pressure, isospin density and the equation of state in the pion-condensed phase and compare our results with recent (2 + 1)-flavor lattice QCD data. We find that the threeflavor χPT results are in good agreement with lattice QCD for µ I < 200 MeV, however for larger values χPT produces values for observables that are consistently above lattice results. For µ I > 200 MeV, the two-flavor results are in better agreement with lattice data. Finally, we consider the observables in the limit of very heavy s-quark, where they reduce to their two-flavor counterparts with renormalized couplings. The disagreement between the predictions of two and three flavor χPT can largely be explained by the differences in the experimental values of the low-energy constants.