  We present the third Open Gravitational-wave Catalog (3-OGC) of
compact-binary coalescences, based on the analysis of the public LIGO and Virgo
data from 2015 through 2019 (O1, O2, O3a). Our updated catalog includes a
population of 57 observations, including four binary black hole mergers that
had not previously been reported. This consists of 55 binary black hole mergers
and the two binary neutron star mergers GW170817 and GW190425. We find no
additional significant binary neutron star or neutron star--black hole merger
events. The most confident new detection is the binary black hole merger
GW190925\_232845 which was observed by the LIGO Hanford and Virgo observatories
with $\mathcal{P}_{\textrm{astro}} > 0.99$; its primary and secondary component
masses are $20.2^{+3.9}_{-2.5} M_{\odot}$ and $15.6^{+2.1}_{-2.6} M_{\odot}$,
respectively. We estimate the parameters of all binary black hole events using
an up-to-date waveform model that includes both sub-dominant harmonics and
precession effects. To enable deep follow-up as our understanding of the
underlying populations evolves, we make available our comprehensive catalog of
events, including the sub-threshold population of candidates, and the posterior
samples of our source parameter estimates.
