  We report the discovery of three HI absorbers toward low-power radio active
galactic nuclei (AGNs) in a pilot HI absorption survey with the
Five-hundred-meter Aperture Spherical radio Telescope (FAST). Compared to past
studies, FAST observations have explored lower radio powers by $\sim$0.4 dex
and detected these weakest absorbers at given redshifts. By comparing the gas
properties and kinematics of sources along radio powers, we aim to explore the
interplay between AGN and the surrounding interstellar medium (ISM). Compared
to brighter sources at similar redshifts, our observations suggest a slightly
lower detection rate of HI absorption lines ($\sim$$11.5\%$) in low-power radio
AGNs with $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})=21.8-23.7$. The
low-power sources with $\text{log}(P_{\text{1.4 GHz}}/\text{W Hz}^{-1})<23$
have a lower detection rate of $\sim$$6.7\%$. Due to the incompleteness of the
sample, these detection rates may represent the lower limits. The selection of
more extended sources and dilution by HI emission at lower redshifts may
contribute to the lower detection rate of HI absorption lines. These detected
absorbers present relatively narrow line widths and comparable column densities
consistent with previous observations. One absorber has a symmetric profile
with a large velocity offset, while the other two show asymmetric profiles that
can be decomposed into multiple components, suggesting various possibilities of
gas origins and kinematics. These HI absorbers may have connections with
rotating disks, gas outflows, galactic gas clouds, gas fueling of the AGN, and
jet-ISM interactions, which will be further investigated with the upcoming
systematic survey and spatially resolved observations.
