  We study the finite temperature phase structure for three-flavor QCD with a
focus on locating the critical point which separates crossover and first order
phase transition region in the chiral regime of the Columbia plot. In this
study, we employ the Iwasaki gauge action and the non-perturvatively O($a$)
improved Wilson-Clover fermion action. We discuss the finite size scaling
analysis including the mixing of magnetization-like and energy-like
observables. We carry out the continuum extrapolation of the critical point
using newly generated data at $N_{\rm t}=8$, $10$ and estimate the upper bound
of the critical pseudo-scalar meson mass $m_{\rm PS,E} \lesssim 170 {\rm MeV}$
and the critical temperature $T_{\rm E}=134(3){\rm MeV}$. Our estimate of the
upper bound is derived from the existence of the critical point as an edge of
the 1st order phase transition while that of the staggered-type fermions is
based on its absence.
