  We systematically inspect the microlensing data acquired by the KMTNet survey
during the previous seasons in order to find anomalous lensing events for which
the anomalies in the lensing light curves cannot be explained by the usual
binary-lens or binary-source interpretations. From the inspection, we find that
interpreting the three lensing events OGLE-2018-BLG-0584, KMT-2018-BLG-2119,
and KMT-2021-BLG-1122 requires four-body (lens+source) models, in which either
both the lens and source are binaries (2L2S event) or the lens is a triple
system (3L1S event). Following the analyses of the 2L2S events presented in
\citet{Han2023}, here we present the 3L1S analysis of the KMT-2021-BLG-1122. It
is found that the lens of the event KMT-2021-BLG-1122 is composed of three
masses, in which the projected separations (normalized to the angular Einstein
radius) and mass ratios between the lens companions and the primary are $(s_2,
q_2)\sim (1.4, 0.53)$ and $(s_3, q_3) \sim (1.6, 0.24)$. By conducting a
Bayesian analysis, we estimate that the masses of the individual lens
components are $(M_1, M_2, M_3)\sim (0.47\,M_\odot, 0.24\,M_\odot,
0.11\,M_\odot)$. The companions are separated in projection from the primary by
$(a_{\perp,2}, a_{\perp,3})\sim (3.5, 4.0)$~AU. The lens of KMT-2018-BLG-2119
is the first triple stellar system detected via microlensing.
