The nature of the finite temperature phase transition in (2+1)-flavor QCD depends
on the quark mass, and the order and universal class of the phase transition
are shown in a diagram called the Columbia plot.
The region of light quark masses in this diagram is not yet fully understood.
We present preliminary results of a three-flavor QCD study using M\"obius-domain
wall fermions to search for the critical endpoint in the light quark mass region
on the diagonal line of the Columbia plot.