  We perform the projector quantum Monte Carlo (QMC) simulation to study the
trion formation and quantum phase transition in the half-filled attractive
SU(3) Hubbard model on a honeycomb lattice. With increasing attractive Hubbard
interaction, our simulations demonstrate a continuous quantum phase transition
from the semimetal to charge density wave (CDW) at the critical coupling
$U_c/t=-1.52(2)$. The critical exponents $\nu=0.82(3)$ and $\eta=0.58(4)$
determined by the QMC simulation remarkably disagree with those of the $N=3$
chiral Ising universality class suggested by the effective Gross-Neveu-Yukawa
(GNY) theory, but coincide with the $N=1$ chiral Ising universality class. In
the CDW phase, we show that on-site and off-site trions coexist and the
off-site trion forms a local bond state. Our work not only illustrates the
formation of off-site trions in two-dimensional Hubbard model, but also raises
doubts about the extent of applicability of GNY model on the attractive SU(3)
Dirac fermions.
