  We study stellar population and structural properties of massive
$\log(M_{\star} / M_{\odot}) >11$ galaxies at $z\sim 2.7$ in the Magneticum and
IllustrisTNG hydrodynamical simulations and GAEA semi-analytic model. We find
stellar mass functions broadly consistent with observations, with no scarcity
of massive, quiescent galaxies at $z\sim 2.7$, but with a higher quiescent
galaxy fraction at high masses in IllustrisTNG. Average ages of simulated
quiescent galaxies are between $\sim 0.8$ and 1.0 Gyr, older by a factor $\sim
2$ than observed in spectroscopically-confirmed quiescent galaxies at similar
redshift. Besides being potentially indicative of limitations of simulations in
reproducing observed star formation histories, this discrepancy may also
reflect limitations in the estimation of observed ages. We investigate the
purity of simulated UVJ rest-frame color-selected massive quiescent samples
with photometric uncertainties typical of deep surveys (e.g., COSMOS). We find
evidence for significant contamination (up to 60 percent) by dusty star-forming
galaxies in the UVJ region that is typically populated by older quiescent
sources. Furthermore, the completeness of UVJ-selected quiescent samples at
this redshift may be reduced by 30 percent due to a high fraction of young
quiescent galaxies not entering the UVJ quiescent region. Massive, quiescent
galaxies in simulations have on average lower angular momenta and higher
projected axis ratios and concentrations than star-forming counterparts.
Average sizes of simulated quiescent galaxies are broadly consistent with
observations within the uncertainties. The average size ratio of quiescent and
star-forming galaxies in the probed mass range is formally consistent with
observations, although this result is partly affected by poor statistics.
