  The co-evolution of galaxies and the supermassive black holes (SMBHs) at
their centers via hierarchical galaxy mergers is a key prediction of
$\Lambda$CDM cosmology. As gas and dust are funneled to the SMBHs during the
merger, the SMBHs light up as active galactic nuclei (AGNs). In some cases, a
merger of two galaxies can encounter a third galaxy, leading to a triple
merger, which would manifest as a triple AGN if all three SMBHs are
simultaneously accreting. Using high-spatial resolution X-ray, near-IR, and
optical spectroscopic diagnostics, we report here a compelling case of an AGN
triplet with mutual separations < 10 kpc in the advanced merger SDSS
J084905.51+111447.2 at z = 0.077. The system exhibits three nuclear X-ray
sources, optical spectroscopic line ratios consistent with AGN in each nucleus,
a high excitation near-IR coronal line in one nucleus, and broad Pa$\alpha$
detections in two nuclei. Hard X-ray spectral fitting reveals a high column
density along the line of sight, consistent with the picture of late-stage
mergers hosting heavily absorbed AGNs. Our multiwavelength diagnostics support
a triple AGN scenario, and we rule out alternative explanations such as star
formation activity, shock-driven emission, and emission from fewer than three
AGN. The dynamics of gravitationally bound triple SMBH systems can dramatically
reduce binary SMBH inspiral timescales, providing a possible means to surmount
the "Final Parsec Problem." AGN triplets in advanced mergers are the only
observational forerunner to bound triple SMBH systems and thus offer a glimpse
of the accretion activity and environments of the AGNs prior to the
gravitationally-bound triple phase.
