The LIGO-Virgo O3 Run and the Multi-Messenger Investigations of Compact Binary Mergers

The third observing run (O3) of Advanced LIGO and Advanced Virgo started in April 2019 and ended in March 2020 due to the pandemic. From the O3 run three catalogs of compact binary mergers, GWTC-2, GWTC-2.1, and GWTC-3, that include also some exceptional events, are produced by the LIGO/Virgo Collaboration. The paper will review the science results about compact binary mergers during the O3 run and the follow-up of gravitational wave candidate events involving the whole electromagnetic spectrum and neutrinos. No confirmed counterpart is found during the O3 run for any candidate. The impact of detected events on astrophysics and cosmology will also be discussed. The paper will also briefly summarize additional multi-messenger investigations involving candidates not initially associated to gravitational events.