Aims. We systematically inspected 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. Methods. 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 our previous work, here we present the 3L1S analysis of the KMT-2021-BLG-1122. Results. 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 )∼(1.4, 0.53) and ( s 3 ,  q 3 )∼(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 )∼(0.47  M ⊙ , 0.24  M ⊙ , 0.11  M ⊙ ). The companions are separated in projection from the primary by ( a ⊥, 2 ,  a ⊥, 3 )∼(3.5, 4.0) AU. The lens of KMT-2018-BLG-2119 is the first triple stellar system detected via microlensing.