  With the isovector coupling constants adjusted to reproduce the physical pion
mass and lattice QCD results in baryon-free quark matter, we have carried out
rigourous calculations for the pion condensate in the 3-flavor
Nambu-Jona-Lasinio model, and studied the 3-dimensional QCD phase diagram. With
the increasing isospin chemical potential $\mu_I$, we have observed two nonzero
solutions of the pion condensate at finite baryon chemical potentials $\mu_B$,
representing respectively the pion superfluid phase and the Sarma phase, and
their appearance and disappearance correspond to a second-order (first-order)
phase transition at higher (lower) temperatures $T$ and lower (higher) $\mu_B$.
Calculations by assuming equal constituent mass of $u$ and $d$ quarks would
lead to large errors of the QCD phase diagram within $\mu_B \in (500, 900)$
MeV, and affect the position of the critical end point.
