  We present results of an all-sky search for continuous gravitational waves
which can be produced by spinning neutron stars with an asymmetry around their
rotation axis, using data from the third observing run of the Advanced LIGO and
Advanced Virgo detectors. Four different analysis methods are used to search in
a gravitational-wave frequency band from 10 to 2048 Hz and a first frequency
derivative from $-10^{-8}$ to $10^{-9}$ Hz/s. No statistically-significant
periodic gravitational-wave signal is observed by any of the four searches. As
a result, upper limits on the gravitational-wave strain amplitude $h_0$ are
calculated. The best upper limits are obtained in the frequency range of 100 to
200 Hz and they are ${\sim}1.1\times10^{-25}$ at 95\% confidence-level. The
minimum upper limit of $1.10\times10^{-25}$ is achieved at a frequency 111.5
Hz. We also place constraints on the rates and abundances of nearby planetary-
and asteroid-mass primordial black holes that could give rise to continuous
gravitational-wave signals.
