  We investigate the phase structure of QCD with 3 degenerate quark flavors as
function of the degenerate quark masses at vanishing baryon number density. We
use the Highly Improved Staggered Quarks on lattices with temporal extent
$N_{t}=6$ and perform calculations for six values of quark masses, which in the
continuum limit correspond to pion masses in the range $80~{\rm MeV} \lesssim
m_{\pi} \lesssim 230~$MeV. By analyzing the volume and temperature dependence
of the chiral condensate and chiral susceptibility we find no direct evidence
for a first order phase transition in this range of pion mass values. Relying
on the universal scaling behaviors of the chiral observables near an
anticipated chiral critical point, we estimate an upper bound for the critical
pion mass, $m_\pi^c \lesssim$ 50 MeV, below which a region of first order
chiral phase transition is favored.
