  Increasingly, uncertainties in predictions from galaxy formation simulations
(at sub-Milky Way masses) are dominated by uncertainties in stellar evolution
inputs. In this paper, we present the full set of updates from the FIRE-2
version of the Feedback In Realistic Environments (FIRE) project code, to the
next version, FIRE-3. While the transition from FIRE-1 to FIRE-2 focused on
improving numerical methods, here we update the stellar evolution tracks used
to determine stellar feedback inputs, e.g. stellar mass-loss (O/B and AGB),
spectra (luminosities and ionization rates), and supernova rates (core-collapse
and Ia), as well as detailed mass-dependent yields. We also update the
low-temperature cooling and chemistry, to enable improved accuracy at $T
\lesssim 10^{4}\,$K and densities $n\gg 1\,{\rm cm^{-3}}$, and the
meta-galactic ionizing background. All of these synthesize newer empirical
constraints on these quantities and updated stellar evolution and yield models
from a number of groups, addressing different aspects of stellar evolution. To
make the updated models as accessible as possible, we provide fitting functions
for all of the relevant updated tracks, yields, etc, in a form specifically
designed so they can be directly 'plugged in' to existing galaxy formation
simulations. We also summarize the default FIRE-3 implementations of 'optional'
physics, including spectrally-resolved cosmic rays and supermassive black hole
growth and feedback.
