  Observations of gravitational waves (GWs) by the advanced LIGO--Virgo
detectors provide us with ground breaking opportunities to test predictions of
Einstein's theory of general relativity (GR) in the strong field regime. In
this article, we summarise the nine tests of GR performed on the new GW signals
included in the third GW transient catalog, GWTC-3. These tests include overall
and self-consistency checks of the signal with the data; tests of the GW
generation, propagation and polarizations; and probes of the nature of the
remnant object by testing the BH ringdown hypothesis and searching for
post-merger echoes. The results from the new events are combined with those
previously published wherever possible. We do not find any statistically
significant deviation from GR and set the most stringent bounds yet on possible
departures from theory.
