2025ApJ...995L..58S

Galaxy mergers are predicted to be a critical stage of merger-driven SMBH growth and galaxy evolution. Systems exhibiting multiple active galactic nuclei (AGN) are an important observational tool for constraining the prevalence and properties of merging SMBHs. Most multi-AGN selection methods focus on the identification of AGN pairs, while few focus on the more elusive triple AGN, which are a prediction of hierarchical growth and natural laboratories for studying SMBH dynamics. These systems are rare; only two confirmed cases exist locally (z < 0.1). Here, we present a new, confirmed triple AGN system, WISEJ121857.42+103551.2/WISEJ121901.77+103515.0, with nuclear separations of 22.6 and 97 kpc, identified within a sample of spatially resolved mid-IR dual AGN. The system exhibits multiwavelength evidence for three distinct AGN hosted in an ongoing galaxy merger. To confirm the nature of this system, we present new radio observations with the Karl G. Jansky Very Large Array in A-configuration at 3, 10, and 15 GHz, providing subarcsecond resolutions, and the Very Long Baseline Array at 4.9 GHz, providing milliarcsecond resolutions. These observations confirm this system as a triple AGN, and the first triple AGN in which all three nuclei host radio AGN.