  By varying the intrinsic initial geometry, the p/d/$^3$He+Au collisions at
the Relativistic Heavy Ion Collider (RHIC) provide a unique opportunity to
understand the collective behavior and probe the possible sub-nucleon
fluctuations in small systems. In this paper, we employ the hybrid model
iEBE-VISHNU with Trento initial conditions to study the collective flow and the
fluid behavior in p/d/$^3$He+Au collisions. With fine-tuned parameters,
iEBE-VISHNU can describe the $v_2(p_T)$ and $v_3(p_T)$ data from the PHENIX and
STAR collaborations. However, for some certain parameter sets with initial
sub-nucleon fluctuation, the hydrodynamic simulations have already beyond their
limits with the average Knudsen number $\langle K_n \rangle$ obviously larger
than one. Our calculations demonstrate that, for a meaningful evaluation of the
fluid behavior in the small systems, model simulations should also pay
attention to the validity range of hydrodynamics.
