  We explore the buildup of quiescent galaxies using a sample of 28,469 massive
($M_\star \ge 10^{11}$M$_\odot$) galaxies at redshifts $1.5<z<3.0$, drawn from
a 17.5 deg$^2$ area (0.33 Gpc$^3$ comoving volume at these redshifts). This
allows for a robust study of the quiescent fraction as a function of mass at
$1.5<z<3.0$ with a sample $\sim$40 times larger at log($M_{\star}$/$\rm
M_{\odot}$)$\ge11.5$ than previous studies. We derive the quiescent fraction
using three methods: specific star-formation rate, distance from the main
sequence, and UVJ color-color selection. All three methods give similar values
at $1.5<z<2.0$, however the results differ by up to a factor of two at
$2.0<z<3.0$. At redshifts $1.5 < z < 3.0$ the quiescent fraction increases as a
function of stellar mass. By $z=2$, only 3.3 Gyr after the Big Bang, the
universe has quenched $\sim$25% of $M_\star = 10^{11}$M$_\odot$ galaxies and
$\sim$45% of $M_\star = 10^{12}$M$_\odot$ galaxies. We discuss physical
mechanisms across a range of epochs and environments that could explain our
results. We compare our results with predictions from hydrodynamical
simulations SIMBA and IllustrisTNG and semi-analytic models (SAMs) SAG, SAGE,
and Galacticus. The quiescent fraction from IllustrisTNG is higher than our
empirical result by a factor of $2-5$, while those from SIMBA and the three
SAMs are lower by a factor of $1.5-10$ at $1.5<z<3.0$.
