  JLQCD collaboration recently started the $N_f=3$ QCD simulations with the
$O(a)$-improved Wilson fermion action employing an exact fermion algorithm
developed for odd number of quark flavors. It is found that this theory has an
unexpected non-trivial phase structure in the $(\beta,\kappa)$ plane even at
zero temperature. A detailed study is made to understand the nature of the
observed phase transitions and to find the way of avoiding untolerably large
lattice artifacts associated with the phase transition.
