  Three-flavor QCD simulation with the $O(a)$-improved Wilson fermion action is
made employing an exact fermion algorithm developed for odd number of quark
flavors. For the plaquette gauge action, an unexpected first-order phase
transition is found in the strong coupling regime ($\beta\lesssim$ 5.0) at
relatively heavy quark masses ($m_{\mathrm{PS}}/m_{\mathrm{V}}\sim$
0.74--0.87). Strong metastability persists on a large lattice of size
$12^3\times 32$, which indicates that the transition has a bulk nature. The
phase gap becomes smaller toward weaker couplings and vanishes at $\beta\simeq
5.0$, which corresponds to a lattice spacing $a\simeq$ 0.1 fm. The phase
transition is not found if the improved gauge actions are employed. Our results
imply that realistic simulations of QCD with three flavors of dynamical
Wilson-type fermions at lattice spacings in the range $a=$ 0.1--0.2 fm require
use of improved gauge actions. Possible origins of the phase transition is
discussed.
