  The rare decay $K_L \!\to\! \pi^0 \nu \overline{\nu}$ was studied with the
dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a
single event sensitivity of $( 7.20 \pm 0.05_{\rm stat} \pm 0.66_{\rm syst} )
\times 10^{-10}$, three candidate events were observed in the signal region.
After unveiling them, contaminations from $K^{\pm}$ and scattered $K_L$ decays
were studied, and the total number of background events was estimated to be
$1.22 \pm 0.26$. We conclude that the number of observed events is
statistically consistent with the background expectation. For this dataset, we
set an upper limit of $4.9 \times 10^{-9}$ on the branching fraction of $K_L
\!\to\! \pi^0 \nu \overline{\nu}$ at the 90% confidence level.
