  We study three-flavor QCD in a uniform magnetic field using chiral
perturbation theory ($\chi$PT). We construct the vacuum free energy density,
quark condensate shifts induced by the magnetic field and the renormalized
magnetization to $\mathcal{O}(p^6)$ in the chiral expansion. We find that the
calculation of the free energy is greatly simplified by cancellations among
two-loop diagrams involving charged mesons. In comparing our results with
recent $2+1$-flavor lattice QCD data, we find that the light quark condensate
shift at $\mathcal{O}(p^6)$ is in better agreement than the shift at
$\mathcal{O}(p^4)$. We also find that the renormalized magnetization, due to
its smallness, possesses large uncertainties at $\mathcal{O}(p^{6})$ due to the
uncertainties in the low-energy constants.
